jrbruc84 You have a couple of choices here,
The easiest one is to go to a body shop and have the fiberglass cutout to clear the tires. This will also work down the road if you go with 17" wheels. ERA does this to their cars when the bodies are built my car went back to ERA for this change. I when for 15" to 17" and they clear with 335's in the rearend and no hit marks.
Not so easy idea is to get a set of custom rims made with the correct back space to clear the body. If you want to go cheaper, they sell back spacers that go behind the rims between rim and rearend housing. They come in different thicknesses. IF you normal drive, they will be ok. IF you are going to drive hard, I would recommend this and do the rims.
Worst case, like I said, the rearend is not square in the body and need to be remeasured and centered. You may still need to have the inner body well modified. I don't know if ERA will sell you the Patch kit. Just need to find a glass man that willnot RAPE you for this repair. When done add 3 -4 coats of undercoating on the body only. This will help protect the paint and stop bullet marks in the paint from debrie kick up. Some guys use foam board and spray it black.
All you can try and do is this to start. Put the car on jack stands, remove the rear shocks,(all). Put tires back on. Put a pump jack under the tires and go through the range of motion and is where it rubs and how close the other side is. You might only need 1/4" to clear the body. Again back to slotting the mounting plate to clear the body. You have 15" wheels for now but after you goto 17" I doubt you will return to 15" except on long cruises over unknow roads. That extra ground clearance can save an
oil pan.
I trailer my car in 15" wheels and race in 17". I went from 335 PIlots(Mich) to 315 Nitto tires. YOu have my number if you want to talk. Rick L.