The bolt pattern is 4 1/4. I bought some high quality wheel adapters to get it to 4 1/2. Since this will be a street driven car, I am not worried about using adapters. I widened the rear fenders 2" each and moved the rear running board brackets out as well. The side splash aprons are still even with the rear quarters. You can't tell it was altered unless you know what you are looking for.
Widening your rear fenders is going to be a terrific plus.
I don't have the IRS in mine, but I do have a Jag IRS going in another project. It too has the 4 1/4" (108 mm) bolt pattern. Very difficult to find a decent wheel to fit.
I have a 10 bolt GM rear end in mine and when I got the car the back tires interfered with the bodywork. Initially, when I changed tires and rims - I went to a 3 3/4" backspace rim, it was the best I could locate. I still had to add longer shackles in the back (leaf springs still on the car) to enable tire clearance.
We are now switching over to a different 10 bolt which will have a triangulated 4 link, ala standard GM, so the old leafs can go.
The Plymouth frame has a taper all the way to the back, and we straightened that out in order to gain some additional clearance for the tires. It looks as though you have tons of room between your tires and the frame.
