IMO no reason NOT to go 4.25", with a 6.535" (BBC +.500") you can use the same 1.32" CH piston they make for any 451 B low deck 6.76" rod combo, everybody makes that piston. a 4.25" with 2.200 journals has BETTER block clearance than a 4.15" stroke with the B/RB mopar journals.

FWIW....the 4.15" stroke came about because it mathematically allowed any RB owner to bump to 493 cubes by boring to 4.342" reusing his 6.76" rods and using an off-the-shelf 400B piston.