New to the A body scene, school me!

Do your suspension setup first. Then acquire the motor you want to use, take a real hard look at the late 80s/early 90s LA 360 Roller Block, pretty much the top dog of production LA blocks. That roller cam setup is a big advantage over standard flat tappet blocks.
Personally I'd just keep the 273 running (maybe swap on a 4 bbl intake and carb and that's it) because you want to swap out to a bigger motor later anyway. More money to build the bigger motor is always a plus.