I had one of those Remington 700-based muzzle loaders, some time ago. Nice rifle. If memory serves, rifling pitch is 1:24"....so well suited to saboted bullet use. I used both jacketed and cast lead bullets. Jacketed: .44/240gr. and 300 gr. Hornady XTP. Also, .44/265 gr. Hornady JSP. This one became my preferred bullet, for deer hunting. For cast bullets, I used either 240 gr. or 310 gr. swcfp bullets. Both, cast from Lee mold. Also worked well. I used other brands of jacketed bullets too, depending on availability. The rifle showed no real preference for one versus another.
For powder, I mainly used Pyrodex RS. Followed by Goex Ffg. Occasionally also Hornady pellets....but preferred loose powder overall, as it gave me more latitude in powder charge adjustment. You can use 3-F (or equiv.), if you prefer. I didn't find any real performance advantage to the faster powder, so stuck to either RS or Ffg.
A good starting powder charge for any of the bullets mentioned, is 90 gr. (by volume), of either Pyrodex RS, or Goex Ffg. Or one of the equivalents: Swiss, etc. This is also a good all-around deer hunting load, particularly with 240 grain bullets. Upping the powder charge to 100 grains, worked well with all bullet weights. Also showed good accuracy and performance on deer-sized game, out to 200 metres.
I don't recall offhand what the maximum charge was for these rifles. You may need to contact Remington for that information. But anything around 90-100 grains should work very well for you, at least to start.
Al