318 or 360?

Agree with both of the above posts. First order of business is to make sure you have the correct mounts for your year chassis and intended engine, in hand. Unless you're willing/capable of doing some fabrication. Too many people get all starry-eyed and build their dream motor and then realize they have no way to mount it in their car- and then it just sits on an engine stand in the corner of their garage. So before even starting, get the mounts for a 318 or 340/360/Magnum to a slant 6 K frame of your year before you even start, unless your plan is to swap to a V8 K frame. Either way, HAVE the parts. The project can go no further if the parts aren't available. The same goes for the exhaust- have the proper manifolds or headers ready to go before you even start. Those are the two biggest holdups on projects like this. Good luck with your project!
Edit- And you do realize the transmission needs to be swapped to a small block V8 unit too, right?