The OE 273 pins were very heavy (smaller ID) so that the smaller dia (therefore lighter) 273 pistons could be balanced the same to a 318 crank as heavier 318 pistons were. a person could use 318 pins since you are rebuilding/balancing from scratch but I think 273 pistons are not avail & even if they were the CR would b too low. Hughes may have something for you but depending on what head there may b a bore shrouding prob. Not much help to offer but wanted to kick things off for you