Another piston option for the 273.

Interesting and thanks! What are you going to do on wrist pins? Bush the rods or bore the piston holes? Have you considered offsetting the pin upwards in this process to help move the piston top up?

Have you looked at the bobweights and where it will end up? It sounds like you are going to be taking a fair amount or material to get this balanced.

The 68 cc chambers sound like the open chamber ones. Maybe you could save yourself some work if you ran down some closed chamber ones. But, since you are a machinist, it may be no real barrier to mod the block & present heads instead.