Get it checked!
First, I recommend that you get the rifle checked out by a good gunsmith to make sure it is in good working condition.
As for ammo, I believe that Remington's factory loaded 300gr hollow point load and 405 gr soft point loads are safe to shoot in Springfields. I've seen some PMC ammo that is marketed for the cowboy shooters that is safe in Springfields. As far as I know Winchesters ammo is safe also. You want to stay away from the loads produced by the specialty companies like Garrett's Hammerhead loads, Buffalo Bore bullets and probably Cor Bon's ammo.
If you reload most of the loading manuals list 3 level of loads for the 45-70. You will want to stick with level 1 loads. Level 2 and 3 loads will likely damage your gun and maybe damage you!
Standard military loading of the 45-70 was a 405gr round nose lead bullet backed by 70gr of black powder for 1,350 fps. The cavalry carbine loading was the same bullet backed by 55gr of black powder for 1,150 fps. They also used a 500gr round nose lead bullet backed by 70gr of black powder for 1,315 fps.. I don't know how this translates with Pyrodex. I've never used it.
Black powder cleans up pretty well with hot, soapy water. There are also several black powder solvents on the market that should work well in a Springfield.
If you intend to roll your own ammo I suggest that you purchase a copy of Lyman's reloading manual. They list loads for lead bullets as well as jacketed.
