I was originally going to build a 383 for my 51, but after reading about block clearancing required (and maybe some filler around bottom of bores) I decided to go with a 400. It is from a running 87 Monte Carlo, 87K miles, so it should just require a standard rebuild. Some good alum heads, EFI and a RV cam, it should make a good engine for the truck to pull a trailer. Probably going with a 5 speed manual and a 12 bolt or 9 inch rear. I am nowhere near actually getting the truck running though.
my .02
