Here is a quick look at some info about the Tall Deck BBC motors. I think? only the HD 427 truck and 366 HD truck are tall decks. Not sure about the 402? The Tall Decks are 0.0400" taller and have 4 ring pistons. We had a fleet of 1978 Chevy Cabovers with the 366 BB. 402 is even better. Right?
I am a Fan. They were lousy route trucks, but I thought they were fun to drive with the 2 speed rear. They sounded good. I saw a 427 BBC C truck admirably pull a sled at the Van Diego County fair once. I also rode in A SD Co Sheriffs unit that may or may not have had a HP 427, from what I can research. I believe it was A 1968 or 69? Chevy Police Special. Growled like an Angry Tiger! I have a relatively quiet 49 state lol exhaust on mine, but She sure starts to moan at speed out on the highway. 3.42/TH400. LOL. BB's are so much fun. A complete 402 sounds pretty good to me for free.
As noted the HD Truck motors are different. If they are too big to fit in a Doghouse? IDK. I'd measure that up or maybe check a long block for fit because you are there fighting motor mounts anyway? I don't know if a BBC G van Doghouse is the same as a smallblock, or not? It is tight. Stock BB Exhaust manifolds are hard to find. You have more room for exhaust with the leaf spring setup. Baddass Van, Man!