100 grainsV of Pyrodex or 777, and a Speer .500dia 325gn Unicore in an MMP Purple sabot. Speer Unicores might be a little hard to find but they shoot great.
Barnes 54x50 275gr Expander is also a really good bullet for the 54s. I didnt care for the 325gr version nearly as much. Needs a much hotter load to get the accuracy i wanted. The Speer though is the best if you can find them. Speer also made a 50 300gr but its actually a tougher bullet. Less expansion but a little more penetration. Not really needed for deer. The Speer 325gr is exceptional at "old school" ML speeds.
That Harvester 54cal sabot is excellent for hot loads. That Powerbelt however is VERY light for 54cal. TC used MMP sabots and its a bit larger than the Harvester. Use whichever fits tight without being a huge struggle to load. The 300gr XTP in them should be fine.