273

Except for pistons, all your 273 parts will fit into a 318 block. A 360 would need much more to make it work. If you go that route, get a complete 360 short block if you plan to keep the 273 heads and stock exhaust manifolds. Or, get a 360 long block if you're willing to play with the exhaust system. You'd also have to modify engine mount brackets, find an oil pan that will work and not rub, flywheel rebalance or conversion to a later car flywheel, bell housing, clutch, etc. A lot will depend on what sort of performance you want, and what your budget is.