Thank you for your replies. Frank Wells, the gunsmith in Tuscon who was doing the rechambering of my ZKK-602 ran into difficulties with three reamers having different dimensions, one 0.048" short, 0.078" long and the correct one. It ended with him declaring he hadn't seen a foul up like this in all his years of 'smithing even on less well known cartridges, which is saying something.
It started when I fired my first cartridge, it had an extra shoulder so it looked to have been chambered short. I then was able to purchase around 80 cartridge cases of original Weatherby .375! I sent him a couple and he ordered a new reamer which came in 0.078" long (these are all base to beginning of shoulder measurements). In the mean time I had traded 3 original cases to a collector in Nebraska for a whole box of early Norma .375 H&H brass that had been fireformed in a .375 Weatherby chamber (not original Weatherby), guess how long these cases were? Yup, ~0.078" long. So using the two reamers and setting the barrel back one turn Frank was able to match the dimensions in the A-Square reloading manual. I used this as a reference because my two main reference books; The Handloaders Manual of Cartridge Conversions by Donnelly and Designing And Forming Custom Cartridges by Howell have different specs. To make sure I was on the right track with the specs in the A-Square book I called Weatherby and carefully explained the problem to an engineer in the engineering department. He told me since (at that time) the cartridge was being tested by SAAMI he couldn't give me any info at all on it, he would tell me that, "according to the dimensions you read to me the re-released .375 Weatherby will have no problems chambering in your rifle". Ok so he told me it was the same cartridge. Either was I have a fine rifle that I have fired around 15 rounds through.
Everything finally came together just as finals were starting at UAF so I had around an hour to zoom out to the range and pop some caps to see what it was like. I like it. I was doing my fireforming using 235 grain Speers with 78 grains of IMR-4350, Winchester brass and CCI-250 primers. I couldn't find Winchester primers in Fairbanks. They pop out perfect. Using the same load with 300 grain Sierra Spitzers is quite different than my .500 A-Square which is the only other large rifle I have. The .500 is a huge PUSH while the .375 is a strong JAB. I liked the difference. I don't know how much my rifle weighs, I think it is around 9 pounds or so.
I had thought of using H-4831sc and try to get 2725-2750 with my 25.1" barrel. I will use some H-4350 and see how it works. Ackley's books all show IMR-4350 as THE powder but we have more choices these days. I am looking forward to the powder companies coming out with loadings.
Ron and Pumba thanks again for your replies and David, wouldn't you know I had decided on this chambering and sent my barrelled action to Wells to only find out some days later Weatherby was bringing the .375 back. So much for my "I just want to have something different" rifle". 