How much tire can I fit?

So it is getting close to tire and wheel time for my '74 Duster. It has a Dana 60 and Caltracs, stock tubs and trimmed quarter lips. I pulled out some new 28x10.5 Mickey slicks mounted on 15x10 Welds and mocked them up. The Welds are 3.5 backspace so I knew that wouldn't work. But I have extra long studs so I can play with spacers. Hey, let's flip them over and add some spacers. Wow, they fit! So it looks like I need 5 in backspacing to run this tire.

All I see are 4.5 or 5.5 backspacing online. I spent a couple hours searching and came up empty. I do remember when I bought some Alumastars I got them in 4 in BS but I would rather not go with a pricey wheel this time. Any suggestions? I could 5.5 and run a 1/2" spacer but I don't know if that is safe.

I am trying to fit as much tire as I can back there. Also, I am sure a drag radial has way less sidewall bulge than this ET Street bias ply does so I may have more room than I think (didn't buy those tires yet!).