Problem with the GM barrel, it's 1". You could very well run out of wood on either side of the forend by the time you opened it from 15/16" to 1".
Early GPRs and others were made in 58, so it's technically feasible to have a 15/16" 58 cal barrel. The question will be whether you can find one or find someone willing to rebore to 58. The world is pretty lawyered up now, and popular "wisdom" is that 15/16" is too small for 58, much less 62. You'll have to have that debate with possible sources.
If I was bound and determined to have a GPR in 58, my first move would be to search out one of the odler ones as-is. Problem there is that the folks that own them love em and aren't willing to turn loose.
Next in line would be to locate a bare 15/16" 58 cal barrel, and simply build the barrel from there. Rebore would be easier and cheaper, but when I nosed around, I couldn't find anyone willing to do it. Times may have changed, but right or wrong, I'm not optimistic.
I solved the problem by picking up a couple of T/C Big Boars in 58. My hunting pard picked up a GM 1" barrel for a TC Hawken, then got a Big Boar. Another pard picked up a Big Boar.
GM isn't turning out replacement barrels any more, at least for the foreseeable future. But 58's for the TC pop up used now and then. If you see one, grab it. Nice backup for you to be able to pick up a TC someday and add that to the barrel, just in case you can't solve the Lyman GPR conundrum.