I've tried and liked Universal for that 255gr. LSWC bullet(commercial cast Magma mold) that is commonly used for 45Colt in the 45AR.
Above bullet is sized at .452
5.5 grains Universal
CCI-300 primer
Loaded into Top Brass cases with an overall length of 1.220
I crimp these bullets with a Redding profile crimp die which is designed to put a roll crimp on the case mouth. I prefer using a roll crimp on any pistol bullet that has a cannelure. I'm not sure if I ever chronographed this load, but I'd be surprised if it was much over 825 fps. in my 4" 625. I do remember that the point of impact is considerably higher than the 200gr. SWC target loads that I usually shoot in that gun. I think you are on the right track looking for a powder of that burn rate for that "heavy" for caliber bullet in the 45AR. I probably tried VV-N340 also, but I seem to recall that the Universal load shot a bit tighter.