A body K member

Don't forget that whatever V8 you install, you will need the matching engine brackets; AND passenger side mounts are different between the 273/318, and the bigger SBMs.

340s work great in these cars.
A hi-compression 4-bbl318 is also a good choice.
A 273 can be a nice cruiser .
But a hi-compression 360 streeter is/can be, borderline overkill.