Biggest tire in Stock rear fenderwells

I don't know OC,

I just put 255-60-15 BFG TAs and Summit chrome alluminum 8" wide with 4.5' back spacing on my '69 Swinger and even with rolled lips I'm here to tell you it is CLOSE! I can only get my baby finger in between the wheel well lip and the tire on the driver's side and I can't quite get in on the passinger side. I swapped out the original 6 leaf springs with new ones but I got 5 leafs. I installed them and it did raise the car a bit but not enough.

Do you have a spring offset kit as well in your car? I just got a 67 Dart that needs some new tires and was wondering how big I could go. I plan to do an axle swap in the future (8 1/4 now, either 8 3/4 or a Dana eventually), but I would like to be able to keep whatever wheels and tires I get the first time. Trying to figure out what size rims I need to start with. I'd like to put street slicks on it, but I'm not sure how oversized I can go on them to fit narrower rims (we had a 69 Charger with stock rims that had something like 275 tires on them and the wheels almost looked like a funnel there was such a difference, lol). I'd like to do 10-12 inch tread, but I don't know if street slicks will put up with as much tread to rim difference.