What is a goodload for a 1 in 20 twist?
Assuming this is a .45 caliber, it is the classic, standard .45/70 twist rate. You should use full-caliber, conical bullets between 300 and 500 grains with 70 up to 120 grains of FFg BP or an equivalent replica powder by volume.
You will probably get your best accuracy with the heavier bullets. My 1/20" twist .45's shoot very well with the Lyman-Gould .457122HP (342 grains from pure lead) sized .451"-.452", so they are a tight sliding fit in a clean bore. They expand into the rifling like a Power Belt on firing. I use a .45 Wonderwad under the bullet, and shoot them "naked" - no lube on the bullet..