G'day BoS, I have a Martini Cadet set up as a switch barrel with barrels in 17Mink and 20/357max. I'm watching this thread carefully as one of my future projects was hopefully to be a 300 Whisper Rimmed using 357max brass. My current project is a 310/357mag with the original barrel.
The thinking behind the 20/357max was to use standard reamers and dies, to eliminate the cost of custom dies and reamers which are quite substantial here in Aus. The chamber was cut by my local gun plumber with a standard 204R reamer run in short to 357max length and a rim recess cut in the face of the barrel. The reloading dies are cheap Lee ones. I trimmed the base off them to the right length. That was all easy and then came the usual problems you find with wildcats. as you discovered 357max brass is really hard and must be anealed before necking down. The next problem was the difference in diameter at the base of the case. The sized 357 brass would not fit in the chamber because of that few thou interference fit. I tried sizing down the solid head on a case but that was a wasted effort. So back to the Gunsmith and had him lap out a couple of thou from the rear of the chamber till the FLS max cases slipped in. Problem solved .I have the barrel stamped 204 short rimmed and its now up and running and slaying rabbits.
I believe from my experience that for a 300 whisper rimmed I would just have a chamber cut with the standard reamer and a rim recess cut to suit the max brass. Them I would use 300/221 dies and a 357mag shell holder. You already have the case forming sorted out- just give you G'smith a couple of your cases so he can lap out the first 0.200 of your chamber to suit your brass.
If you decide to go the custom reamer route then PTG are really good to deal with. There was no problem at all getting the 17Mink reamer from them.
I have assumed from the CBE bullet that you too are in Oz.
Keep us posted
Cheers