I have 29x14.5x15 Hoosier cheater slicks that I run at 11-12 psi at the strip on a 68 Firebird. Of course, correct tire pressure depends on many things including vehicle weight distribution, application, etc.

As for the shaking problem, I would bet on wheel balance being most likely followed by drive line. I had a similar shaking with a previous set of tires that set in > 70 mph but was smooth as silk to 100+ after rebalancing the rear wheels.

Two cents,

Chris