The car was probably originally a mono leaf. Previous owner installed new multi leaf and installed a "lowering block" (???) to lower the rear end. It's pretty micky mouse, and the tires are rubbing aginst the fender wells, and the shocks are hitting the leaf springs. Additionally the U-Bolts are WAY too long, and they're grade 5.

I have a set of traction bars. If I put the traction bars and remove the lowering block, from the pics I have seen, I will 2 sets of U-Bolts for each side... one for the front of the traction bar, and the larger ones that go around the differential.

What are the length of the two different U-Bolts that I need.? (69 Camaro)

use the ones that are there and just cut off the excess.

But, shouldn't they be stronger than a grade 5? I would think they should be and, if I have to get new ones, I might as well get the right length / diameter ones from the beginning. Also, I don't have any of the "shorties" that go in the front of the traction bars, so have no clue how long / what diameter they should be?

grade 5 is fine, and you only need 1 set. The traction bars replace the orignal lower shock plates, so there's still only one thing to attach.