I obtained one of these at a recent gunshow together with some shells made from reformed 45/70 cases
Has anyone ever loaded for this cartridge and if so did you use regular 8mm projectiles

Bad, slug that bore before you do anything drastic. I have a vague recollection of an article from the time that those rifles were common on the US market that said several fiddling changes were made during the life of that cartridge.

The only one I ever owned was a good bit bigger than .323 and had an obviously unshootable bore so it rapidly got rebarreled to .45-70. Made a mighty good .45-70.

I've got RCBS dies and a 375 barrel would be easy to get and have the darn thing chambered to 38/56 Win?

Other options include 303 British, 7.62 Russian, 30-40 Krag, 348 Winchester. IIRC the 30-40 and the Russian come pretty close (at least in rim diameter).