Cartridges of the World lists the bullet diameter as .514 with a 42" twist but does not give a bullet weight for an original cartridge. I would say anything from 350 to 450 grains will work OK. That would be the Lyman 515139 and the 515141. There are other molds available such as Moose and Accurate.

The 50/70 Maynard is not the same as the Gov't round and is quite scarce. I've only seen one. The one you bought makes two that I know of. I think Rocky Mountain is going to be about the only choice for cartridges. a fireformed 348 Winchester with an adapter ring may work but the brass may be too thin at the neck.

Good luck with it. I'm expecting the 32/20 today. The snow has held up delivery. Gotta love those Maynards!

The nice thing about a mold that is too small in diameter is you can send it back and have it taken out a few additional thousandths, as needed.

I have a 50-70 Maynard 1873 model and RMC made cases up for me easily.  I shoot a 450 grain .515 bullet and it is extremely accurate.  The barrel starts out as 1 3/8" octagon and goes to round, but is still heavy.  The full length Stevens scope helps my eyes see the target, but I was shocked when I saw the accuracy.  I took it hunting and shot a deer with it a few years back.  That slug really packs a wallop, even at low velocities. 
Bruce

I did some research yesterday on the bullet for your .50 cal Maynard. I found lots of references to .50 bullets but no dimensions. Then in one book I stumbled on a passage that stated that brass and molds could be obtained from Dixie Gun Works. It might be worth a try. Also Dixie used to sell a lot of Rapine molds maybe a look through a Rapine catalog might help.

40 Rod

Bruce, I had forgotten about that beast of yours. You sent me a picture of it once. Do you still get this way once in a while? I haven't seen you since Tulsa. I'll be in MI again in July for the Alma shoot.