There are many threads on this issue. 273 to 318 is the easiest swap of them all! In short:
USE the 273: Timing cover, indicator tab and balancer. Eng. mounts, Dipstick, Pulleys and brackets, exhaust manifolds, oil pan and pickup. 1 thing to watch is: if going to a 318 with airpump ports, the 273 drivers manifold will intefere on the exh. port pad. You'll have to grind that down slightly. Its thin, so maybe some off the manifold, and "little" off the pad.
