I heard the Remington Ultra Mag series was the Imperial mag series ( they bought the patent ) out of Osoyoos bc, if that is the case then they had a 360 Imperial Magnum and it would be very similar to the 358 UMT you are talking about.
They are similar but the 35-404 that Bevan has the reamer for and the 30-404 that I designed and built 2 rifles for both pre-date the Imperial magnums by some years. It is my old hunting buddy that Ted alluded to in his previous post and I was there when he bought the rifle from Bevan and was there many times to witness it's effect on game, a seriously effective cartridge, with recoil approaching that of the 378 Wby. It was this cartridge that prompted me to design and build my 30-404, it worked out perfectly as Luke would form and fireform cases from 404 brass and then I would take them and neck them down to 30 cal and develop my loads. A wonderful co-operative venture. Back then I could order brass out of Huntingtons and have it in about a week or so, Bell, A-Square, and occasionally RWS if we were lucky. Luke also managed to get about 40 cases marked Parker-Hale but we had no idea who actually made them, suspect they may have been Kynoch.
Them were great days of wildcatting, scrounging for cases, load development, scrounge for more cases, search for slower powders like H570, more load development...........a lot of fun, waiting impatiently for new reamers to arrive in the mail, barrels already threaded and contoured........waiting.......the good old days, for sure.
OP, anyone with a 300 RUM reamer and a 35 and 416 neck and throater could chamber those up for you, it ain't rocket science. In a CZ magnum or 602 Brno you can get 4 under in the mag if that matters to you and you have lots of action and magazine length to play with your OALs. The Rem and Sako actions are shorter, but can be lengthened with a fair bit of work and the Mod 70 it shorter yet and considerably more work again to lengthen.