New build wheel,tire and brake questions

[joesbruiser]

My son and I are about to start a new build.We built a 66 c-10 that got him on powerblock that has been sold and a new 69 c-10 with a 67 front end, the top very mildly chopped and A- pillars laid back for this build.
We plan on using a Porterbuilt front and rear dropmember,arms etc....
He has picked out what wheel he wants to run I am just not sure on backspacing and tire size and if we should just run 6 lug all the way around with standard disk or spend the money and go with a set of 5 lug axles 5 lug front hubs and then run willwood brakes front and rear.I know there is a big money difference.
The wheel he has picked is not what I would have picked but its his truck.He wants to run the KMC Slide in black.I am thinking 20x8.5 up front and 22x9.5 on the rear.What backspacing and tire size should we go with if we go this route?