Dan -
I would have THOUGHT that the ERA 427 S/C body would have the same or possibly MORE tire room than the 289
FIA body. I would expect that Bob would be best qualified to answer that, in reference to ERA bodies, anyway.
Here are a couple shots of the tire/wheel clearace of my right rear tire from the rear with the 305/50x15 BFGs on a 10.0" wide PSE
FIA knock-off wheel. Sorry I didn't think to put a tape measure up there, but, as you can see, there is plenty of room on my car.
I THOUGHT I had similar pics with the old Goodyears, but could not find them.
regards,
Jeff