need some help with wheels

[sixty8c10]

I am about to order my wheels, and i have a couple of questions.

First I should give you the rundown of whats on it/ going on it.

68' C10, six-lug, LWB, 2 and a half inch drop spindles with disc brakes and and bags with the lower cup cut down to one inch. My plan is to order 20x8 or 20x8.5 wheels for the front with 5" back space, and mount either a 255/35x20 or a 255/40x20, will these fit in the front or do I need more backspace?

Out back will be a c-notch 2" blocks and bags. I am planning on a 20x10 with 4.7" backspace and a 265/40x20 tire. Will these fit?

Also is anyone on here a Bonspeed dealer?

Thanks for the help, Toby