I can't speak to this from first hand experience, but since no one else answered, Lyman's manual(45th Ed) shows:
155gr gas check 15 - 18gr unique
23 - 27gr 2400
24 - 28gr 4227
173gr gas check 14 - 17gr unique
22 - 26gr 2400
22 - 27gr 4227
193gr gas check 14 - 16gr unique
20 - 27gr 2400
20 - 27gr 4227
I remember when this was a new cartrdge, someone wrote that they'd had good results with cast bullets in this calibre, although it looks to me like an awfully big case. Also, lyman's old(1950's or early 1960's) cast bullet handbook listed a variety of slower powders for cast loads in .300 holland & holland which would give you a good starting point.
Hope this helps. Ron