First congratulations on your new project.

If it's a 318 automatic, chances are it has a 904 rather than a 727. These are a good transmission in light duty applications, but not as heavy duty as the 727.

The 318 is what is called an LA engine (small block)and shares the same bellhousing bolt pattern as the 340 and 360 engines.

The 426 HEMIs are based on the B/RB (big block) motors and require a different transmission as both 904 and 727 have an intergral belhousing/case design.

Check on E Bay and other places on the net and I think you going to find that a HEMI in any codition is WAY WAY out of your budget.

Personally, I think I would get the engine in it running if you can and see what kind of shape it's in. From there you can figure out what way to go. A cheap swap for these cars has always been a 383/400/440. Donner cars can often be purchased for next to nothing and will go a long way in turning the car into a hot rod.