I'm cross posting this, so some of y'all may see this in my build thread/instagram/SVTp thread.
I took it upon myself to find a better way to build the IRS brackets for a foxbody. Some of us are aware of the dilemma, but it seems slightly gleaned over due to the perpendicular bolts taking most of the attention. See below for 99-04 cobra rear IRS upper brackets bolted to a foxbody's stock quad shock mount holes.
After measuring the hell out of everything, I've determined the rear quad shock bracket hole is exactly 3/4" lower on a foxbody over an SN95. As you can see in the picture, this isn't exactly a "bolt-on" affair for fox cars, and will require some work. Everyone seems to focus on the vertical bolts that go into the trunk, and for good reason. But ultimately, there is a better way to properly build rear IRS brackets to fit foxes and SN95 chassis cars without having to even touch the frame.
I started with 1/4 inch steel plate. I had some left over from an old Chevelle frame, but any 1/4 mild steel works. I drew out a template and started cutting.
This what I came out with
Not all brackets are the same. I had to cut a groove into the others sides steel plate to properly fit against the IRS brackets. But I then jigged it up
Once it was welded, I took an angle grinder to it on the inside and out to clean the welds up and ensure a flat fit against the frame.
It's difficult to see/take photos of, but I marked the center location of the hole with two lines on the frame prior to drilling. I then mounted my brackets, got them square and even, marked the location, ensured it was 3/4" lower than the center of the hole above it, and drilled a 1/2" hole. Factory quad shock holes are threaded in M12-1.75 (thanks Stuart) so a 1/2" hole suffices. If you're lucky and nail the location, it bolts on immediately.
Naturally these aren't finished, painted, or completed. But with both hole locations being present, I didn't have to weld next to a tank full of gas, cut my frame, make the brackets fox pattern only, or do anything sketchy. These will be able to come off and bolt to any 79-04 car I own now or in the future.
Next update I will have the brackets taper like factory to get rid of the excess hanging below the frame, and my solution for the vertical bolts. This is just how I chose to do it, and I feel it can't hurt to add some more steel/stitch welding to a spot welded bracket anyway.