"drop in" small block

I've used the motor mounts in 360 swaps, not that difficult. This is really pretty straight forward, especially if you stick with a 318.
318 in same spec will have about 50 more horsepower. Both in 2bbl form, 273 = 180 gross, 318 = 230 gross.
If you want "touch of the throttle" throw back compared to the 273, the 360 would give you the greatest grin.
Seriously, dropping a 360 in where a 273/318 sat is really pretty straight forward. If it's a car 360, just balance the flex plate (or buy one balanced) and your done. I put a 360 in one of my Dusters I have now, and used a flexplate I had balanced.