I'm pretty sure that the reason there are so many questions on wheel (size, width and b/s) and tire size, is that there are soooo many combos. Too many.
You already have the tires. So, there's no question there. Seems like you have all the suspension set up...
I don't know from Jack about "modular spindles"....
You question seems to be about whether the Boss 338 8.5/10.0 20's will work, right?
Well, here's my set up....See how close it is to yours:
Boss 338 20's; 8.5/10.0
Tires are different 245/40's and 275/45's
Basically the frame and box are '71 Fleet Short
ECE 2.5" drop spindles
ECE 2" Springs in front
ECE 6" drop springs in back (no C-Notch)
Shock relocator kit including and KYB (or whatever they are called) shocks
ECE adjustable panhard bar (if that matters...it did to me for centering).
Soooo.....The REAL QUESTION is about the Boss 338 b/s. On my mfg date, they came in one b/s only for the 20's. I had to use spacers to get the rears out away from the bed side of wheel well.
I chose to go with 1" spacers. Good ones. At the time, the b/s Boss was selling was something like 5.23" for the rear. If you dug around on the forum, you would see that Boss came
out with different b/s more recently than my buy. The complaint on those was that they seemed to have added material to get a smaller (in inches) b/s and it did not create a deeper rim
on the outside....(the polished surface in mine).
I have no idea what the 8.5 x 20's b/s is. They are all they had at the time, too.
Subsequent to that first fit, I bought some 1.5" spacers for the back and moved the 1" spacers to the front. That's only because I wanted mine to be "closer" to the outside wheel well...
...wider stance...Different than what you are after, I take.
So, I'll look for pics. But it won't really matter.
You already have the tires. And, you want to buy Boss wheels. So, if there's any choice in b/s, and, like someone else stated, the folks you're buying them from can't tell by your stats,
either run or buy the wheels. There's no science. It is often a guess. About fit...And we're not even getting into the art of the wheel/stance ratio yet.
Like I said, there are as many questions as there are trucks on the road. Just buy the wheels and take a shot. If they fit, you'll be surprised how much you like them once you pay out the bucks.
The first three pics are when she first came home from the shop after I laid under her for three plus years. The one with me leaning on her has a minor P/S (turn signal elimination to see
if I wanna go with the 67-70 front bumper).
The last one is yesterday after I prepped for taking her to the paint man.
Point is, the first three are with the 1" spacers in the back and none up front. The last one is with 1.5" and 1" spacers, rear and front, resp.
Hope this helps.