Call JES. If he hasn't cut the chamber then ask him to cut it to 357 Magnum. Some rifle chamber reamers (I prefer Clymer's 357 Max reamer) are available with cylindrical throats. All mine have been 357 Mag chambers that were either rechambered with a Clymer 357 Max rifle reamer or were properly throated with a cylindrical throating reamer.
Bellm has experience with monoblocs.
