I've built a bunch of 400 block motors with 3.750 stroke up to 4.300 stroke crankshafts, all but the first one where sonic tested by me after having the first block crack the #1 cylinder wall on the camshaft side of the cylinder wall while breaking in the cam on a engine dyno it was right at .035 thick after prying apiece out of it while I having it bored out for a ductile iron sleeve
That block was not salvable due to much core shift on the drivers side
What I have found in the past on bore size is the lack of a good set of piston rings after you get above 4.380 bore size 4.440 bore size rings are available but not many stock blocks will be safe at that size
My first pump gas 400 stroker motor (511 C.I.) I built for myself ended up with four ductile iron sleeves in it due to deck damage between the #4 and #6 cylinders due to a blown head gasket torching that out, #1 and #7 had to much pitting in them to clean up at 4.375 bore size so I had the shop sleeve all four at the same time due to not being able at that time to find another block in SO CA back in 2001
That motor made a bunch of power and took a lot of abuse by me on the track and street on pump gas, it had a set of ported big valve 906 heads with 9.25 to 1 compression ratio to start with and it made 612 HP at 5500 RPM and 644 Ft. Ls. at 4500 RPM with a low deck six pack. I ended up swapping heads several times and crankshaft stroke once to 4.300 stroke (517 C.I.) to make it make more power, it ended up with a set of Indy SR M.W. intake ports with a 400-3 intake and a 1050 CFM Holley Dominator, that combination made 727 HP at 7000 RPM and around 680 Ft . Lbs. of torque with 10,78 to 1 compression using Oregon 91 octane non ethanol pump swill. That combination ran a best of 9.993 at 134.+ MPH corked up with the air cleaner on in my old street Duster weighing 3450 Lbs. with me in it, it ended up being way faster than I expected
Test, test and test some more, don't be afraid to try something different to make more power My current S/P bracket 400 cold weather block that I bought form AndyF on here ended up only needing being bored to 4.350 to straighten out the bore That motor has pushed my 1970 S/P chrome moly tube chassis Cuda (2750 lbs with me in it) (517 C.I.) to 5.50 at 120.+ MPH in the 1/8 mile and 8.886 at 150.+ MPH in the 1.4 mile so far
IHTHs