For a more powerful load, I'd just use a full case of something like H4895 - filled to the base of the bullet with only a few thou of compression. It should shoot accurately, too. This is more or less what I did with my .458 Alaskan with lead - shot MOA at 100 meters with 500gr. Lee gas checked bullets - straight WW alloy at 2,000fps.

I've shot TrailBoss with 500 and 525gr. cast bullets from my .45/60 Sharps. At 800 to 900fps vel. they print 1 1/2" at 100 meters from it's 18" twist. It's possible your 500gr. bullets have something wrong with them to cause keyholes. Are they undersize? That will cause problems. Cast bullets should be a minimum of .001" oversize to shoot well. Gun's vary somewhat in what they want for cast. The 500gr. + will actually stabilize from 14" of twist, right to 22" of twist at those low speeds - and on up to whatever speed you can get. Stabilization is NOT the problem.