Biggest tire on 1969 dart

So from what I see on all posts I have read

A 15" x 8" rim with a 5" backspace you have to roll or trim lip
And I'm not sure about what clearance is left on the inside

With a 15" x 8" rim with a 5.5 backspace clears the lip
But not sure what is left on inside

One post said he tried a 15" x 8" with a 5.5 backspace and it touched the leaf spring
And springs where in stock location and mine are moved in 3/4 " per side

I also seen 1969gts used 5.5 back space
But he don't say how the fitment was and I see he has not been on site since 2010
I tried to send him a measage

Any thoughts