I was measuring rear clearance on my '80 2WD Heavy Half last night, your numbers look pretty much the same. My springs are inboard from the wheel tub by about 1.5", I have 6.25" BS to the wheel tub with a bit of a safety factor. The inner width from the top of the wheel tub to the inside of the fender is a bit less however, are you planning to lower it? With a bit of tire bulge on a 12wide rim + clearance, even with the tubbing to match the springs - I think you're looking @ 13.5-14" of section width. If you lower it until the fender lip is ever below the top of the tire, I don't think it will go. I only have 12" from tub to the lip and that's at the center, even with tubbing 1.5" - the section width on 12's would leave me about 1/4" either side. Put a bit of weight in the box and take a corner I think there would be rubbing, this pic is with 235-75-15 Arctic Alpin winter tires on stock rims.
With stock tire diameter/ride height, you could put any width on - the openings are so cavernous. Of course then the tire is sticking out which I don't enjoy too much, but I been having a hard time finding steel fender flares which are big enough. All the new dually trucks have plastic fenders, there's nothing I can find for repro other than stepside fenders which are insanely $$$. Does anyone know of someone making plastic, bolt-on dually fenders for a 73-80 ? I would like another 6" of body around the wheel opening, but all the steel fender behind it has to be cut out for clearance - so the fender/flare can't just glue/bolt on to the wheel opening. This is why I'm thinking dually/stepside fender, probably need to cut the fender to match wheel tub and then weld on new fender that is big enough to match the hole.
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