10.5:1?
I’d go with 6’s.

I don’t know where you’ve been shifting, but I’d try a couple passes shifting at like 55-5600..... see what happens.
Make note of what the rpm is crossing the stripe.

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads