[CC69Rat]

5.5 " Backspacing would fit nicely.

Most of the guys that run 20's on these trucks run a 10" wheel with 5.5" BS on the later rear end housing.

a 295 tire is going to be 11.61" wide. 295 / 25.4 = the width of the tire in inches.
the next number will be the aspect ratio of the tire. So above the guys mention a 50 series tire.. or 295/50/15.

Take your width -- 295/25.4 and multiply times your aspect ratio. (295/25.4)*.50 = 5.81"

This gives you the height of the sidewall in inches. So we know we have a sidewall on the top of the tire and the bottom of the tire. 5.81*2+15 (<~~ your wheel) = the overall height of the tire itself.

(295/25.4)*.50 * 2 + 15 = roughly 26.75" tall. It all really depends on how the truck is lowered (if any) and how you want the tire to fit in the wheel well. Do you want it to tuck under the fender, .. sit flush with the fender, etc. 275/60/15 10.82" wide and 6.49" sidewall, 28" tall.

Just for reference a 295/40/20 will tuck under a fleetside with 5.5" Backspacing. BUT, remember the 40 series tire will not have as much section width.. (the balloon effect of the sidewalls because they are shorter obviously) a 295/50/15 will look more 'puffy' than a 295/40/20.

Not assuming you don't know any of this, .. just trying to help.
Later.