what rod ratio do those 250 have? Pretty positve with the 125.3 rod recomended on a 68 cub your max safe rpm is not 10k. Probably 9 to 9.5
As far as trickeds comment of them shaking apart thats a bunch of bullshit. Maybe in a spaghetti frame with a bunch of flex they are popping epoxy. But mine ran 3 years of dune racing before i had to rebuild it and not due to case epoxy problems.
I would try and contact brandon from what was wildcard racing. He has real world experience on the salt, he lives in utah, and was an engine builder. Hes not really doing it anymore but its worth a shot.
What size tire you run? I want to run it through my calc to get primary,secondary and trans ideas.