273 rebuild

you can use a 318 crank in a 273 they interchange in fact many people put 273 cranks in 318's because most 318 cranks are cast and early 273 cranks are forged, just like the good 340 cranks, so I would recommend keeping yours, as far as valves the stock size are great for a 273 in fact if you try and install larger valves they tend to hit the cylinder wall, a stock cylinder head rebuild with unleaded valve seats should do the trick, the hardiest part to find are going to be pistons as 318, 340, 360, pistons can't be used, high compression 273 commando pistons would be a nice choice,
try calling EGGE http://egge.com/