My dad and I put a '70 396 in his (now my) '65 SWB C10.
It bolted right up, though we had to hack and slash the headers that were on the El Camino to make them fit. (The days before ready cash.

)
Manifolds would be my choice now. (Old age creeping.)
I would make sure your radiator is up to the task.
The stock three speed lived a long time, as my Dad only ate TransAms, Mustangs and Porsches from a rolling start!
Good luck, and have fun with it!
Mike