10.5:1 Pistons for '66 273 Block w/ '67 920 Heads

When I built mine back in the 90s you could get TRW hypereutetic pistons. You could contact them and see if they will still make them to order.

To answer part of your question, the 273 is the only engine with that particular bore size -- no other pistons will fit. Don't bother looking at 318, 340, etc.