Siamese Mausers are strong actions, and the fact that they are set up for rimmed cartridges gets a lot of attention for a 45-70 build, but there are other options if you really take your time and figure them out. The major caveat with a Siamese Mauser is magazine length. And before you start you need to ask "what am I going to do with this rifle?" so you pick an appropriate cartridge. I found a Siamese Mauser action but it's missing the magazine assembly (the owner has a standard M98 magazine attached and was surprised when I told him it was a Siamese Mauser).
8X50Rmm rim diameter is .550", so you have quite a few options as far as rimmed cartridges. Of course, you can go to 45-70, so we will kind of gloss over that one. You could also go with 30-40 as you mentioned, 405 Winchester (I think this one would be cool), 444 Marlin (a little loose on the bolt face, but it would work), just about any .532" belted or beltless magnum, or one of the many 40-caliber or 44-caliber BP target rounds from Remington, Winchester, Sharps, Ballard, etc. If you are so inclined, you could chamber it is 30-30 or one of its offspring (219 Zipper, 219 Wasp, 25-35, 32 Special, 375 Win, etc), or one of the wildcats based on 30-30 brass (357 Herret and such) There are numerous British rimmed cartridges (flanged to the Brits) if you are looking to build something unusual like a 375 Flanged H&H, although you would be well-served to make sure you can get brass before you do so. There are also numerous wildcats that would be interesting: 416 Barnes (based on the 45-70), 450 Alaskan (based on 348), 458 American (based on shortened 458 Win Mag brass).
So what are you wanting to do with it?
