An L60 14 had a section width just over 11" wide and was about 27" tall (not that short!). Basically a 275/60/14. It would have been a bit narrow for a 10" wide rim, and with "hardly any" backspacing it was probably even with or stuck out past the quarter. But that's not an issue with air shocks, since there would have been very little suspension travel and the car probably sat pretty high in the back.
You can fit a 275 on a Duster/Dart Sport/Demon without any modifications at all even on a lowered car. Springs in the stock location, no trimming on the quarter lip (unless the axle isn't centered well). Just have to get the backspacing right, and that depends on what axle you've got. SBP A body 8 3/4's are wider than BBP 7.25's and 8.25's, and of course an A-body 8 3/4 with BBP axles is wider still. A 275 is the widest tire you can run without modifications IMHO, it's the largest tire that fits with stock equipment and without trimming. If you want to go as wide as possible with the stock wheel tubs, you can run 295's, but that requires a 1/2" spring offset and trimming back or rolling the quarter lip. Do that and a 295 is pretty much the widest tire that will fit and you have to get your backspacing just about perfect to do it if you're lowered any.
I:soapbox::thumbsup::rolleyes: