I'm debating on which block to use for a stroker. Is it better to use a 400 or a 440 block? I have a set of 440 Source Heads (unported version) and a Mopar .590 cam that I'd like to use.
I do have a 440 intake, but it is an older Weiand 7512 (Rated for 3000-6500) and a Holley 850 DP. Pretty sure the carb will be to small and will have to get another one. I have both 400 and 440 blocks that would require the same amount of machine work. Engine will be strictly a race motor and wouldn't be turned more than 6500. Trying to get a 3200 lb car to go 10.20's.

In theory the 400, shorter stiffer cyl, smaller dia mains mean little more material stiffer block plus bigger bore to start with. BUT can put a larger stroke in a RB.
Unless you plan something bigger than 520 IMHO I'd say 400, but later on if you get a aftermarket block use your rods and crank new pistons and build a 572 or bigger.

Your probally going to need a better set of heads and a better cam to acheive your goals How much compression and fuel are you planning on using?I'm in the middle of building a motor similar to your planned motor, 400 block bore to 4.375, 4.250 stroke with a set of unported Source heads, I've already told the customer we are probally going to need a better set of heads to reach his goal of 700 HP on Oregon 91 octane pump swill I've built several different versions of race and pump gas Mopar stroker wedge motors, to reach 700 HP with 511 C.I. or larger C.I. motor you need a set of M.W. size ports that will flow at least 340 CFM at .700 lift on the intake side
It is all doable depending on how hard you want to work and how much money you want to spend

Would like to be able to run a mixture of race gas/pump gas so don't want a motor with to much compression. How much better are other heads compared to the Stock 440 source? I see they offer a Super Stealth version and a CNC version. Who else makes a good head? Are they any out of the box heads that would be good?

if it were me spending my hard earned coin i'd be buying a set of Eddies or if my budget could stretch that far, some Indy lil EZ or Sr's. i'm not impressed with the Stealths.

I use a 440 block with my 493 as I like the fact of the better intake manifold selection for the RB block. And mine is a pump gas street car that I knew I would only race a few times a year at the most so I was not worried that the 400 block has the stronger low end. I went with the basic Indy EZ heads as I felt they have more potential if I want to step it up some down the road and I like the fact that the exh ports are in stock location and you dont need offset rockers. I also use 440Source D-dished pistons to get pump gas friendly 10.6 comp and I built good quench in it. I use a custom cam from Dwayne Porter thats .585 and .592 lift with 264 & 270 @ .050 on a 110 LSA. I have it in the eng on a 106 ICL. I used the 440Source 4.15 stroker kit and run the Indy dual plane intake with an 850 DP carb. Its a street car that I love to drive and the cam gives it the idle I love and makes decent power with good driveability. At just over 3700 lbs its run 10.70's in street trim just like I drive it as I usually drive it to the track and race on 92 pump through the full exh system. At 3200 lbs you should be able to build a nice pump gas eng to get you the 10.20's you want. And honestly I have not really gotten after my car as with my bad knees and back I just dont take the time to go after the tune as I have been happy to just leave it as it is and drive and race it like that. If I dropped 500 lbs and set it up as a race car I feel I could hit 10.20's without much trouble. Good luck with yours. Ron

Either block will work, 440 is a little easier to find intake manifolds for as well as headers. You might want to double check on both before you make a decision.

A 440 block with a 4.25 crank will get the job done. Get the heads ported so they flow 300+ at .600 lift. The .590 cam might get the job done. You only need 600 hp to go 10.20 at 3200 lbs so that is fairly easy trick for a 505 engine.

The 850 carb is fine for 600 hp. You need 1.4 cfm per hp so 600 hp is 840 cfm.

The CNC porting from 440Source should work fine or send the heads to Porter Racing Heads and have him fix you up. The Weiand intake should work okay at 600 hp.

Biggest decision is compression ratio. I'd probably shoot for something in the low 10's if you want to drive on pump gas. You can race on a mixture of unleaded premium and race gas or mix with Hot Rod 102 or something like that. If it is a race car only and you're willing to buy race fuel then increase the compression to 12:1 with a flat top piston and run race gas.

There are a bunch of tech articles that cover similiar engine builds. Take a look at my 505 article with a big flat tappet and slightly ported Edelbrock heads. That engine made about 650 hp and it would be very similiar to what you're trying to build.

http://www.moparmax.com/tech/2011/vi_8-flat-1.html

Since you have both a 400 block and 440 block. I'd have them sonic checked and use the best one. I've had three checked and two were good one was marginal.

A couple of 750 carbs on a tunnel ram help to the tune of 30 or more hp, if that interests you. I had good luck with an 80s vintage TR intake, then ported it and gained an additional tenth by porting it out to bigger runner size.