Opinions

Like post #11, it isn't trivial. You need a different motor bracket on one side. I think your steering linkage will hit the oil pan, though a strategic mallet blow can fix that. You can't swap a 273 or 318 pan onto a 360. Might be easiest to keep all front pulleys, accessories, and timing cover from the 273. The damper and balance weights might be an issue. For auto trannys, you need an adapter ring to fit the rear crankshaft hole. Don't know about your manual tranny, but research the throw-out bearing bore.

A 318 might be easier. I would look harder at a 5.2L Magnum engine (318 ) since that gives a roller cam, cheap fuel injection, better ignition system, serpentine belt. They were fine in trucks, so should make an A-body move fast. Much info on that. Ditto for a 5.9L, but the oil pan issue and worse mileage. Don't know what manual tranny issues would arise. Many have used both Magnum engine and auto tranny (to get 4 spd overdrive), but requires reworking the tranny tunnel in the car.

I bought a set of 0.030" over flat-top pistons for my 1965 273 for $200 on ebay. It took waiting to find them. That is ~2x normal 318 or 360 pistons. You can also buy the HiPo "domed" pistons from Egge, but >$300. I also got a 4 bbl aluminum intake, also w/ waiting (1964 & 65 are unique). If you can wait, you have more options.