rodwha you will come to find out quickly that each and every muzzleloader, even those of the same model and caliber will shoot best with a different load combo of bullet/powder/powder charge. If you're like most of us on here who own several MLs you will find it imperative to maintain a log as to what rifles shoot what combo best.
I've had 1:48 twists rifles that shot patched balls fantastic while others shot conicals well.
With your Great Plains Hunter with its 1:32 twist I believe a good starting point would be somewhere around a 300 gr .429" or .451" bullet with appropriate sabot and assuming it is a caplock, some Pyrodex RS or FFg black powder. Start with 80 gr and work up in 10 gr increments to sell what load shoots best. Remember to swab between shots for the most consistant results. Once you find the sweet spot for that bullet/powder and find that it does not meet your satisfaction, then move to a different bullet or powder or even a different sabot. Your Lyman will shoot great for you once you find what it likes. But its a process of elimination/experimentation.