Biggest tire on 1969 dart

-- Hello
So I hav a 1969 dart with a. 8 3/4 sure grip
The leafs hav been moved in 3/4 per side
What's the biggest rim I can run
Any way a 15 x 8 with a 4" bs will fit or
Is the biggest going to be a 15 x 7 with a 4.50" bs --
-- Move the springs in to the sub-frames. - This is My 69 Valiant.