77mm and 9500rpm isnt particuarly working hard so im sure that would be good but I would be inclined to go for the 90mm stroke and still go to 8500rpm though

Loads of XE engines out there at 86mm making monster power at way beyond 9000rpm, so 8500rpm on a 90mm doesnt seem a big ask at all.

Depends on how bad those rod angles end up though of course!
Can you not gain a few more mm for longer rods by moving the pin up in the piston, espicially if you are going high comp which you obviously will be?