Ill check in a couple of days, but I means its not a strong interference fit with the ring/hub, some very small gap it is I guess.

You wrote about the same spacer, @TKOPerformance, 

TKOPerformance wrote:

My '89 GT with SN95 Cobra brakes required the same spacer, and that's how I secured it. 

I assume you mean the ring, or did you mean the 1.5" spacers that I also wrote about?
 

The issue with the early Galaxie rear axles is the stub axle is alot shorter than Mustangs had. There was more protrusion on the later axles.

Most 15” wheels will clear the cobra rear brakes. It must be the shape of that particular wheel.

Do you have enough fender clearance for that thick of a spacer?  It might not be necessary if you find the location of the interference. On Al’s 65, we had one side that would not rotate and found the part number raised letters cast onto the inside of the rim were barely touching the caliper. Grinding them flat gave the needed clearance.  Find the exact source of the interference and go from there, unless you just want to move the wheels outboard.  I would think a 15x8 would already be a tight fit on that car, but I don’t know your wheel’s backspacing.