a T 5 can't hold up to a strong 305, there's a reason GM never bolted them to a 350... ever.
Are you racing your big block, or just want a kick ass strong motor?
You should have enough torque to not need real deep gears. A normal old 4 speed stick, or a Th400 if you are sick of shifting, would be idea with a 3.07 gear. You'll cruise at 2500 or so.
