70 duster tire size
I've got 15x7 cop rims on my '74. Should fit your '70 just fine. I've got 225/60/15's on mine, you could definitely go larger in the back, TONS of space back there. Front still has some space, it'll depend on how low you are. You can go wider than 225, but as you can see you won't be able to go much taller than a 225/60/15 without modifying the bottom of the fender.
I've got 15x7 cop rims on my '74. Should fit your '70 just fine. I've got 225/60/15's on mine, you could definitely go larger in the back, TONS of space back there. Front still has some space, it'll depend on how low you are. You can go wider than 225, but as you can see you won't be able to go much taller than a 225/60/15 without modifying the bottom of the fender.