I've have a .38-50 RH and have been using the Lyman 40-70 seating die (unaltered) and the neck flare die with a custom made button.  I have a 38-50 full length sizing die that I used initially to form from both .303 Brit and .30-40 Krag brass.  I have to neck size the .303 brass after each use as the case walls are thinner than the Krag brass.  Up to now, I've only shot black powder but may try smokeless in the future.

when I made thee cases for my two 38-50RH, one on a Hepburn, the other on a CPA
Hepburn chamber was done by Ron Snover, the 30-40 case was big about 1/2" from the rim, Ron said to use a 303 Brit die cut off the top to size down the case, I fired the case then annealed it and fired it again, Cream of wheat and Bullseye, then trimmed to case length, in my two rifles , the case inside dim. was ok, even better for shooting Paper patched bullets, both rifles had 1-12 twist barrels, Hep had Shilen, CPA Green Mountain
I only shot BP in both rifles at the time, can no longer shoot BPCR, sold The Hepburn, Will post the CPA, at a later time
I made a compression die using a Lyman body, other than that don't need much to shoot it with acuracy
FWIW 1 1/2F worked best, .060 poly, and .060 cork together both GG and PP
HTH