Ok, we have a 58 Miller/Allin rimfire conversion for the Springfield and a .58 Berdan conversion for same. The center-fire Berdan came in two lengths.
The rimfire .58 Miller takes a .585" bullet, is .628" at the base, .620" at the neck, has a .709" rim, case length of 1.193" (measurment difficult and variable due to extreme crimp in some instances), and an overall length of 1.701".
The centerfire .58 (Berdan) musket cartridge was larger in rim and head but smaller at the mouth than the Miller, so the cartridge conversion info for the Berdan is no good for the Miller. The .577 N. E. can be machined for the Berdan, though is evedently a touch to large in diameter at the head to turn down for the Miller in case you were wondering. The .58 Remington Carbine is a case very similar to the .58 Miller rimfire, and it is suggested to solder 5/8" tubing a .577 case head, anneal, size, trim and chamfer (no turning) to make the .58 Remington cartridge, such would work for the .58 Miller chamber as well along with machining down the .577 rim if you were desperate.
28-gauge CHAMBER is .626" x .614 with a .688 rim, should be perfect for the .58 Miller if cut to length even if the rim is a touch small. No idea on the rim thickness of the 28 gauge versus the .58 Miller rimfire. Hooe that helps, it appears you are on the right track with the Magtec shotshell brass.