Thats a great find. I'm sure you will love that gun. I personally would try a conical in the 465 to 495 with 70-90 gr powderif its a .504.
If it's a .451 I would go with a 460 gr. with 70-90 gr powder .
If you decide to try sabots i've had great results from Dead Center sabots in my .451.with a 100 gr of powder. The fun part is finding the load it likes. Congratulations. Don't forget to post a picture of it and then of the deer you shoot with it.