Trapper, You're not going to get a great deal of response on this, if for no other reason there be few if any here who have had this little beauty in hand and heard the boom.
I know bupkis about the Spencer Rifle but appreciate your interest in such things. I do know a little about BP cartridges and the good news is that they are not terribly cranky about loading for general applications. Cartridges of the BP era were typically named by caliber and charge weight. EX: .25-20 Single Shot, .32-40, .38-55, .45-70 etc. The last number is where you focus for load data,assuming you are using a bullet of appropriate design and weight. So, given the .56-50 Spencer...
you might start looking
here . Or
here. Not terribly complicate this. Somewhere around 350 grains of soft lead, properly fitted your chamber/bore, add somewhere around 40-50 grains of 2F or 3F BP and rock on.
Smokeless loads, well, you're on your own with that most likely. No matter, you'll have more fun with BP anyway.
Best to you,
Dan
PS: If you click on the individual images in the second link they will present in larger format. Click again and you will often be forwarded to a web page wherein the picture originated and find yourself in the middle of a gun looney discussion on the topic.