318 build up

I would use your forged crank from the 273, it has the right pilot shaft for your 4 speed, plus it's stronger. You say your using the heads, so I would just do a re-ring kit for the rest (rings, bearings, oil pump, gaskets, timing chain, lifters and freeze plugs). I doubt you will need a new cam, although you can check. Pistons are not a "must", only a performance upgrade. You said you did not want a race car, so I would just do the basics and save money.