1980 gmc original vs modified HELP!!!

[1972BackInBlackC10]

i have a 1980 gmc sierra classic i purchased from the original owner, the truck only has 35,000 original kms (im canadian no clue what that is in miles lol)

the question is i have a small block that i built and spent way too much money on for my 67 chev, i have decided to go with an ls in my 67 and would like to put the small block in my 80 gmc but at the same time i kinda want to keep it original so im torn between the two

any ideas or opinions???

truck is 1980 GMC sierra classic heavy half
350 4 barrel th400
12 bolt differetial
2 tone cream and white
brown interior with tilt and cruise
no a/c or power windows/locks