I have some 5mm spacers so they are out there. What you can do is try hardened 1/2" flat washers (Home Depot or Lowes has them) and you can buy 1/2" id round hardened shims. Those will come in thicknesses all the way down to a few thousands of an inch so you can really measure the clearance issue quite accurately with stacking them up until the wheel clears.
Washers CAN be problematic since it is possible that even if you have the stack up equal on every stud it could still be off and the wheel spin off center. Now on the back that is less of an issue BUT still could be an issue.
On the front you would feel it right away in the steering wheel because it would wobble.
At least if you sart with the flat washers, it will give you a very close approximationtion to what the thickness of the spacer should be.
As far as how much thread you have engauging, you will need to measure that once you have the spacer set. !/2" is about the minimum that you want.
Rear studs on a Pantera are not readily available in other then stock lengths. You don't want to have to do them if you don't have to.
The Pantera race cars that I have seen look like they have been converted to SAE 1/2" studs which you can get from almost any race supply shop. The rears are special. I don't know of any other car that uses the Pantera rear studs.
It's possible that ARP may have something in the catalog that would work if you need to go that route. Here's hoping that you don't.
Looking at the interference on the ID of that wheel, that looks like the caliper adapter is off a little on the bolt center lines? IF there is clearance between the OD of the brake rotor and the top of the caliper, you may be able to remake the adapter and bring it in 1/8" or so and that would fix your issue there.
That is not an unusual circumstance on race cars where the caliper is "adjusted" so that the brake pad is contacting the rotor at the maximum area. Production mounts are often off that mark by sometimes up to 1/4".
It may also be possible that if you loosen both the adapter and the caliper and tap them down with a rubber mallet while the bolts are in but loose that you will be able to gain enough clearance without going further.
The other thing that you could do is just let the caliper "clearance" the wheel itself as long as it is basically just wearing off the paint. You DO NOT want the caliper to "score" the wheel completely around it's circumference. That would be encouraging a "failure" through that line as if you cut it on the lathe.
Remember though that magnesium is porous and needs to have a sealer over it so the air molecules do not pass through the porosity when the wheels are cold. That's not a joke, that's for real. They won't hold air unless they are coated.
The back of the spokes has to be done with a spacer. Do not cut those wheels. That may sound good providing you have a lathe that you can get a 15" wheel on? Those are the big mama lathes and few shops have those around anymore. Those went out with the Navy battleships like the Missouri.