First timer here so expect some rookie moves. I have a brand new M code 4.49 bore hemi block and all the peripherals parts to build it except for the pistons. My goal is to build a great smooth running street hemi with 9.4:1 compression that purrs like a kitten. It has a factory 3.75" stroke Eagle crank and stock length Eagle rods, A Mr Six Pac hemi cam and lifters, mopar performance heads and factory spec rocker arm assembly. I am trying to determine what type of piston to go with or if I should change this config because most of the time the 4.5 bore blocks go to build a 528 or 572 based on stroke length of 4.14 to 4.5. I would sincerely appreciate some coaching here from folks who have experience with building or setting up great smooth running street engines.

Thanks and Merry Christmas from moparmatthews

How about sell the crank, rods, heads, cam and rocker arms, and buy a complete stroker kit, massaged quality aluminum heads, hydraulic roller cam and quality rocker arms? Put a nice EFI set-up on it, and tell the well-regarded builder you want it tuned to be a smooth running, pump-gas burning street motor that is like Dr Jekyll and Mr Hyde.

Pistons shouldn't be a problem, you'll just need to get semi-custom ones to get the correct bore size and compression ratio.

Talk to Paul at Gray's Automotive in McMinnville. He has built a lot of Hemi's over the years and he can get that engine together for you.

I was where you are now a few years ago, I went with the 4.5 stroke crank, and got a killer deal on a set of the MP 9.5 to 1 pistons, I wish I had more compression now, there is a set of 12 to 1 MP pistons on ebay now. I started with stock rockers, went with StageV rollers, hydraulic roller does not work easy in a Hemi, the lifters are too tall. I went with a mech. roller with pushrod oiling. Those new heads that flow like 400 out of the box seem like the ticket. I would love to have those heads and fuel injection....Tim

The 426 street Hemi motors can use more compresion on todays pump gas with no problems than any other motor out there with iron heads if I was you and money wasn't a real big problem I would sell the other parts and buy a 4.25 stroke crank with BB Chevy journals, a set of BB Chevy type 7.100 long H beam steel rods and have a set of pistons made that would have a true 10.5 to1 compression ratio. I built a local customer and freind a motor just like that with a custom ground hydraulic roller cam and he loves it, it has iron heads and a dual plane single four barrel intake off of a crate motor with a Holley 850 double pumper, it is in 1968 Charger with a Dana 60 with 3.73 gear and short(28 inch tall) and a 727, full interior not cut up. It is a hoot to drive, responsive as all get out IHTHs
EDITED. The BB chevy rods and the 4.25 stroke crank won't require any grinding on the cylinder walls to clear the rods and crankshaft If you where using the same stroke crank with Mopar rod journals and mopar stock type rods you will have to grind some on the cylinder walls and also on the oil pump pickup boss to prevent them hitting
I've built a bunch of Mopar 426 Hemi and B and RB wedge motors with the BB Chevy journals and 4.25 stroke, they work great in these motors

A hemi needs compression to run well, I would bump that up or you will be outrun by a good running 340.

I'd sell some of those parts and go at least 4.5" stroke or MORE! You will forever kick yourself if you build a motor with only 3.75 stroke. A big MILD motor will live forever and make MONSTER Torque that will scare the feces out of you. Just my $.02Dave

Like everyone else says, put in a stroker crank. You will also need a lot more cam. That Mr. Six Pack cam will run out of steam way to early with a big motor. Hemis like to rev. Don't hold it back with a 5000 rpm cam.

I've been there and done that so I can completely relate.

The torque from a stroker motor rocks, but too small of a cam and it will run out of air waaaay to soon.

Depending on if you go with a flat tappet solid lifter cam or roller you can get a custom ground cam for your combination for starting at roughly $200.
Obviously a roller cam will cost more.

It sounds as if you are just looking to use your 4.5" bore block in place of a stocker 4.250" bore block, not wanting any headaches associated with the build or needing any tuning afterwards, just a smooth running cruise night car type of engine that is street driven and not beat on. Is that right? Not a performance minded stroker in other words? Can you tell us your expectations for the car/engine?

I went with the Go Big or Go Home attitude. Its all the same money to build a 426 as a 572....

This is what I think he is looking to do.. just a
cruiser that runs on the low octane... will never
see the track and wants a low rpm engine... its
not my cup of tea but it may be his... when I had
my hemi it was years ago and it took a lot of tuning
to keep it right... now days if I were you I would
put injection on it

The OP has clearly stated his goals, says he is a rookie, and has asked for advice.

Andy F has the best suggestions. And, like Andy, I would also encourage the OP to find an experienced Hemi builder and follow his advice on how best to achieve his goals.

I agree that if you're new to the game, then getting your feet wet on a relatively expensive platform to build, like the gen 2 hemi, then having an experienced builder guide you through the process makes a lot of sense( or just have them build it for you).
All that being said, the smallest thing I would build to make use of that 4.50 bore block would be a 540(4.5 bore x 4.25 stroke).
If he already has the 426 top end and is just looking for a cruiser type build, I would just reuse it all. Clean up the heads, get a decent cam........ 600-650hp on pump gas should be a no brainer, or more...... Depending on just how mild the finished combo is.

A 4.5 bore with 3.75 crank is still almost 480 cubes so it isn't a small engine. Since the OP already has everything except the pistons the lowest cost option is to just buy some pistons and put it together.

Obviously a stroker kit would add torque and allow a milder combo to make power, but the OP might not have the cash to buy another rotating assembly.

What exactly are you going for? HP/et expectations? Unless you specify you'll have recommendations from a stock 426 to a 1000 hp monster. To me I'm guessing you want something like quickd100 has, streetable, big cube, low RPM deal (even still his puts out 800 hp in a very mild package)

He does state it...in his opening ... last sentence..smooth
running street engine

a 480 hemi with that cam will be pretty smooth, very strong through t he RPM just not a real high RPM. If I was in his boat I would build what I have but run at least 10.5 compression since you got a hemi that is detonation resistant as well as the big bore allows you to get a little bit of quench. Something as complex as a gen II hemi you don't need to throw in more variables like stroking it when it is not needed. It is not like what he has is very valuable so he can not sell it to finance a stroker anyhow.

Very true Mr P but again it's different for everyone, 800+ streetable HP (quickd100) is a much different path than 500 ish (stock style 426), just wondering what end of the spectrum he's on

You will always get different variations and opinions on a build.Find someone with hemi experience and discuss your goals and parts on hand and expectations.Gather all the information for the intended combination,choose the correct parts and spend the money for the right parts and spend it once.Many times someone will start a build and get too many opinions and buy and build a engine changing things many times and not get the results and end up with a lot of extra parts and expense.
Get a solid game plan and stick with it.A good basic plan for a 4.500 bore is to build at minimum a 528" using a 4.150 crank.