I think the 402 was the available big block for pickups in 1970, but it should be no problem to drop any 396/402/427/454 under the hood using mostly factory parts.

I have two 454s, and the better of the two was free. Look around a bit and you should be able to find one pretty cheap too.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...