I'm beginning building my 440 into a stroker engine for 100% street use. My builder recommended a MOLNAR crankshaft and rods. From what I have read they seem to be good quality but my question is can I use MOL4264250MC6F which is a 4.250 stroke hemi 8 bolt crank in my 440 block or do I have to use the 4.150 6 bolt crank? The price is the same but I get a few more cubic inches with just a little more grinding. Is there any differences between the 2 cranks beside the stroke being longer and 8 bolts instead of 6?

Just make sure you go with a 2.2 rod journal crank. Most 4.15 are 2.375 and in that case you may as well go 4.25. Nothing wrong with either stroke. A 4.15/2.2 would be almost no grinding. And you could still usea flat top piston depending on what head you use. some will say you may as well go 4.25 and they may be right. But to be honest 12 more cubes isn't going to do much. I personally think the longer the stroke the harder it is on the bottom end. But that's arguable. You probably won't go wrong either way.

The 4.25 stroke is the way I would go. The torque should come in maybe 100 rpm lower, which isn't much, but hey, if you don't spend anymore $$,,,,,,,,
The actual cube difference is 2.6 percent, worth while to me. You just can't beat cubic inches in a street motor, even with restrictive heads. At 512 to 540 the torque gets brutal and gearing can be reduced to match. I would consider about a 3.55 gear with a big motor because street traction just isn't that good anyway.
The 2.2 rods and bearing journals take a bunch of stress out of the bottom end due to a fair amount less weight. The pistons are a touch lighter as well.
Weight comes off the crank shaft as well when the crank is balanced. Out dated sfi eight bolt flex plates can be had cheap, if you can find an expired one from a guy like me that puts few runs on his stuff and needs to replace for SFI cert anyway. Don't be afraid to look into a 4.5 crank either. The rpms come down and the stress on the parts is even less because the rotating assembly in a 440 block can use a 7.10 chev rod and a much shorter piston. I have run 4.5 stroke with iron heads and peak power was 5200 rpm or their abouts with a tunnelram and 590 cam.

My 440 has the 4.25" stroke crank and 7.1" rods (440 Source kit), and the only clearance grinding required was the oil tube boss. I used the 17cc dish pistons 0.030" oversize, so the engine is really 505", not 512". I threw some mild ported stealth heads on it so compression is about 10.3:1.
This was built for the street, Comp XR296HR cam (really mild in this size engine, but idles at 1,000 RPM with a nice sound) 236/242 @ 0.050 duration. I put 3.54:1 gears and an overdrive 5-speed transmission in the car. 1,500 RPM in OD, is 51 MPH, and I don't need to down shift to accelerate.

A 4.250 crank with 6.80 rods makes a nice combo. Diamond has shelf pistons for that combo. You'll just need to buy a new flywheel or flexplate with the 8 bolt crank.

Everyone should get rid of the 8 bolt crank option since the extra bolts aren't required but for some reason the mfgs keep making them. Most of the crank guys have gotten rid of the Mopar rod journal size so I guess that is at least a start.