8 1/4 sure grip questions

PCDuster makes a good sugestion. Junk yards should have a few Dak's in them.
Gear ratio is stamped on the ring gear or on a tag on the rear cover bolt.

I'd do a 3.55 for the best of both worlds. 3.91's make Hwy. driving a bit of a drag. For a short time....OK. 1/2 max for me kind of thing unless it's to the strip or show/meet.