Not trying to further dump on you here than others have already...........

but for all the extra $$ need to run rpm's........

I'd change out that old obsolete gearbox and get a splitcase Casale to run a low rpm RB (440-500 c.i.).

It's hard to widen an rpm band more than anyone else has yet in history. Developing the "cake and eat it too" set-up means lots of trial and error gear and prop testing.

Engine wear goes up exponentially with rpm.

As FL states famous Big Daddy used to say, "it's better to dial back and win consistently than to run out on the ragged edge and break often".