Unless you are really worried about "correct, an option is to just ditch the spacer.
As you seem to be buying a repro H-pipe.... go to a muffler shop, ask for an off cut of the right size (usually 2" IIRC) pipe. Cut your H pipe weld in the new part, done.
Or find one that will cut/weld for you. Mark the cut and tell them how much you need added. You do have to replace the long studs with shorter to mount the pipe, unless they have miles of thread, mine didn't, bare in the middle.
I did mine at the shop that does my body/paint, I prepped it up and as I can't weld worth a damn one of the guys did it in about 5 minutes tops... zero cost other than a bit of my time.