Sorta surprised no one mentioned this one:

I had the same thing happen on a 73 charger I had back in the 90's. Drove it that way for 5 years- as a Daily Driver. Finally bought a Ramcharger so I pulled the engine/trans in the Charger for a rebuild. Dropped the pan and- #3 main bearing- the one with the thrust surfaces- the tolerance on those thrust surfaces is supposed to be like .010" (going on memory here guys), mine was like 1.0" . Yeah, nearly not there. Oil was just flowing out of there. Chewed up the crank pretty good. So, out it came and it was rebuilt, etc...etc..etc.

If you're going to go through all the work of swapping engines might as well find a later model 360 magnum. Same dimensions but with the added bonus of more cubes and much better heads/cam/block/etc. You'll have to use the timing cover and cam and eccentric from an earlier engine to use a mechanical fuel pump, but an inline electric will tame the gasohol blues in a street car anyway. 360 magnums are dirt cheap and in abundance at most junkyards. Just look for any 250 series truck and you'll find one.

IF it has to be a 318 the 5th ave suggestion works- the 85 and ups had roller cams and the good swirl port heads. They can be adapted to about anything. But given the choice, I'd look for the magnum.

Just my pennies,
Good luck with it.

CC.