QA1 Suspenssion 335 tire?

[keefro]

I've searched the forums but have not found very good results.

The pictures below are of my 1971 GMC with stock 12 bolt rear end, QA1 coilovers front and rear with a 4/6" drop. I'll have some custom 20" wheels made and am trying to decide on tire size. The 335/25R20 installed on the wheel fitment tool is off my Z06. I'll run a taller 35 sidewall at a 29" diameter and the tubbed inner fenders (wood bed makes it easier).

The speedway motors tool says I can run a 20x12.5 with 7.75 backspace and +25 offset (1").

Math:
(12.5" wheel + 1") / 2 = 6.75
7.75 - 6.75 = +1" or +25

Has anyone gone this wide?
12.5" wheels?
+25 offset?