Well it's all up to you. You already have the 273. If the cam and lifters are still good you could reuse them. Pull it, send it off to the machine shop, bore it put in some HP pistons, new bearings, gaskets, new timing set, reuse the E4 and run with it. If you find a 318 you will need to do the same to it. If you have a automatic you might need to mess with the crank snout to fit your converter. (or buy a new converter) There can be quite a few differences between the 273 you own and a old and newer 318 that you will need to deal with.
