Question About Wheel Interference on MK4 with IRS

[BUDFIVE]

Folks, I have a question about wheel interference and spacers for my MK4 Roadster with IRS
I have no experience with spacers and my search results were more related to coupes.
In my build thread weekly update
https://thefactoryfiveforum.com/showthread.php?47734-BUDFIVE%92s-Roadster-build-in-Central-Texas-347-Ford-and-ITS
I described interference between Powerstop 13” 2015 Mustang GT rear brakes and my FFR Halibrand 17x10.5” wheels. It seems to be interfering on the caliper corner edge near the outside of the wheel mounting flange
The front 17x9” clear the Powerstop 13” 2004 2 piston front brakes fine.

On the rears I tried washers on the lug nuts:
-with 1 0.105” washer the wheel still barely rubs.
-with 2 washers (0.210”) the wheel spins freely.

Should I use a wheel spacer?
Is 0.25” enough, or 0.5”, ie how much clearance is needed?
I plan to run 315/35-17 and 255/40-17 tires
If I run rear spacers will the tires hit the fender outside edge?
What about how it will look?
Will I want to run the same spacer up front?
Should I try to pinpoint the interference and grind the caliper?

Enough questions, I’ll shut up and listen.

[DW66]

Grind a little of the top of the calipers. I would personally do this (and did) before running wheel spacers. You only have to grind some off of the nubs that stick up from the top of the caliper. This doesn't hurt anything except paint. If you do this then there are no worries with tire rub. You will want to check wheel weight clearance as well.

Dave

[michael everson]

same as Dave. Grind down the outer radius.
Mike

[David Williamson]

The Coupe body is wider than the Roadster so many of us use spacers to move the wheel out for a better fit with the body.
David W

[BUDFIVE]

Thanks Dave, Mike and David. When I get back next week I’ll look for the nubs on the outer radius.
Is there a minimum clearance or just no contact? I think these cars are light enough the wheels shouldn’t flex much, but I don’t know.

[BUDFIVE]

Per the above recommendations, I isolated the interference between the rear caliper and 17x10.5 Halibrand replica wheels. Some grinding with a die grinder as shown in the pictures below-a couple with the ground surface and one after repainting. Only ~1/16” had to be removed. Thanks for the suggestion y’all.
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