318 or 360? That is my question...

::UPDATE::

Hey Guys,
As an update, I’m purchasing a 74’/75’ 360 from a gentleman out of Troy, NY complete from carb to pan (minus a starter) for $450 (newer crank, rods, pistons, valves, springs, 288/454 cam, performer intake and 600 Holley). As far as what I’ll need for the swap, I want to make sure I don’t miss anything before I pull the old motor, does this look accurate?
1.) Order an oil pan since it was out of a truck.
2.) Throw new mounts in just to be safe.. (stock ones from the parts store should work? I’ve read different things)
3.) He says he’s 99% sure it is internally balanced. Is there any possible way to know for sure and should I still order an externally balanced flexplate? What type is recommended? (3 Speed Automatic trans)

Anything else I’ll need to do? Thanks for the help guys.