I'm using GPM pistons in my 750. They came to me from Andover Norton via OldBritts. 1,000 miles on the engine now and no problems. No smoke, no oil consumption, runs good!
Those stupid oil rings did make it difficult to install the pistons into the cylinders though. Next time I'd be inclined to look for NOS Hepolites with the three-piece oil rings.
Debby