I don't have any of the Hornady 300gr to make a comparison to. Dunno about powder, you'll just have to pick one and work up a load from existing trapdoor data, I'm partial to Hodgdon Extreme powders, H4198 and H4895 are two that I've used most, but I've also used H322, but most of my H&R 300gr loads are in the upper levergun levels and/or bigger bullets, 350gr at 2000fps and 300gr at 2400fps.
Reloder 7 is a real popular powder with the Levergun boys, pick one you have and work with it.
The Hornady manual shows 8 powders to 1800-1900fps for their 300gr HP... SR4759, Viht N-120, IMR4227, AA5744, Viht N-130, RL7, H322 and IMR3031.
Tim
http://www.stevespages.com/458_1_300.html