Probably a 9in 28in tall slick,and you would need some longer shackles and air shocks to raise the rear.Then w/ tires that tall get a lower gear.
I don't know a thing about slicks.. I do know that air shocks are highly likely to tear out the shock mounts under the back seat on these cars if not modified/strengthened.
I've run 28 x 9 MT's before without any modifications to rear ride height. You just have to be sure to get your backspace right on your wheels. I currently use a 1/4 inch spacer between my wheels and axles to get the correct clearance. I currently run 26 x 9 Hoosiers, and have no issues at all. I've also run MT Drag Radials without issues. They hooked just as well as slicks on my car...they just don't last for me.
If it helps, my 70 had stock shocks up front to keep it on the ground and old air shocks in the rear with no air. Leafs with Cal-Tracs and drag radials and it stuck like glue.
