Need help with Wheel fitment

[D-n-Tyke]

I have been searching and reading for the last couple hours and am more confused then when I started. Hoping someone can help me with this. I am posting in the truck side as the traffic over here is much higher (and assume the suburban guys look over here just as much as the suburban side).

Found a set of Boss 338 for sale but am not positive they will fit on my Suburban. I know the only true way to answer this question is to measure my suburban but I am converting it and haven't installed all the suspension yet (still waiting on some parts).

Truck:
1969 C20 Suburban
Rear End = 1971 C10 5x5 lug pattern
Font End = Stock control Arms
CPP 2-1/2 Modular drop spindles
CPP 5x5 brake kit
Looking at going to a probably around a 3/5 static drop.

Wheels I am looking at:
Rears = 20x10 5.0 backspacing
Front = 18x8.0 4.0 backspacing

I am pretty confident the front will fit no problem it is the rears I am a little concerned with. I have read that people have run both 5.0" and 5.5" backspacing wheels on their trucks. Can someone help me out. I would like to try and go get these wheels today if they will fit but don't want to waste my time (2hr dirve) and money if they won't fit. Thanks for your help.