An inexpensive Lee mold for the 50-90 is a 500 gr FN plain base:
It is big enough to size to your .512" and only $26.90 direct from Lee.
I recommend you try it UN- sized before considering sizing. It should be big enough as cast for a slide fit to chamber and require no sizing, just a tumble lube. Cast bullets that fit correctly should take 1-2 pounds more push than a drop in fit to chamber. Make a dummy round and ink the bullet. chamber/unchamber and a slide fit will be identifiable on the top exposed bearing band. A slide fit that you can manageably chamber is the best fit you can get. Any less subtracts accuracy potential.
You only need one good fitting bullet to make your rifle shoot well and it should with this Lee mold.
I know you want a 600 gr but re-evaluate your reasons and get the Lee, it will kill the largest Grizz easily with a very modest load.
Lyman recommends 10 powders for a Cast 515 gr. in 50-70, any of them at START for the Lee 500 gr. is a wonderful hunting load: I'd choose this one:
*RX7 25 gr.
BHN 15 Lyman #2 alloy is recommended to match the load level. Your velocity will be slightly higher and your pressure slightly lower with the Lee 500 gr. compared to a 515 gr.
*verify Lyman Cast Bullet Handbook 4th Ed page 231.