I wouldn’t even think about modifying a spindle. But you can get 8 lug disc conversions with drop spindles. Here is a link to the Performance Online version. I’m sure others make them but that was what I found first using Google.
https://www.performanceonline.com/19...-8-Lug-Deluxe/
If you want 3” or more drop you will need it. You can do a GM front conversion and 2” springs with no troubles and not affect your load carrying capability.
You are correct on rims, though. The way around that is run narrow wheels and tires in the front for sure. The GMT 800 Alcoas are 6.5” I think and they hav 4” backspace.
I did a disc conversion on my 70 GMC 2500 with factory parts. I ran Alcoa 7” classic polished rims with the holes. I ran 265 75 16 tires.
Do you have a coil or leaf rear? That is probably a bigger concern getting a drop in the 4-5” range with leafs. I have a half ton short step with leafs that doing a flip on netted 7” drop. I didn’t want to spend the $$$$ for drop leafs. On my truck I am quite happy with the result, but I won’t be hauling anything real heavy....