1966 valiant rim width with spacers

Nope I think you nailed it :(
Going down to 7” is fine if it means no mini tub or leaf relocation.
So 7” with 5.5” backspace could work?
That still puts the rim right at the lip of the wheelwell. Mount a tire and you're slicin' rubber.
Even 6" of backspacing on a 7" wheel only gives you 1/2" clearance on each side according to your measurements- and that's before you add the thickness of the wheel lip (a 7' wide wheel is closer to 7 1/2" wide). The sidewall of a mounted tire will protrude more than that. Plus you need to accommodate sidewall flex when cornering... I think you're looking at 6" rims, but I don't know if you can get enough offset on a 6". Those adapters are killing you here.
Now this is all based on your measurements- 8" of wheelwell clearance, lip to leaf. 1 1/2 to lip, 6 1/2 to the leaf from the adapter face. Double check your measurements, because I know guys run 7" wide rims on early As all the time- but I think those adapters are going to limit your ability to find rims with enough backspace.