The problem with a 425hp big block is a 350hp smallblock will kill it. 425hp isn't a challenge(unless you want lots of idle vaccum). The factory MP 435 hp crate motor is a simple template for you to follow. I'm not crazy about the 318 block builds,just find a decent 360 block and be done with it. You don't need the 4" crank but if you insist on a street converter it would make for a quicker ride. If I was starting from scratch, a 360 magnum or pre mag hyd roller block,hyd roller cam 240ish@.050 on both sides 108lsa,.530-.560 lift,10 to 1 compression,edelbrock magnum heads,rpm airgap 750dp 1 3/4" headers. Very easy to make a fun but mild street car.