A real '73 trapdoor Springfield in good condition is a treasure. It is worth a good sum of money if it has not been bubbaized and is worth a Large sum of money if it is in original '73 configuration without upgrades by the Army.
The best reference on loading and shooting the Trapdoor is the book "Loading Cartridges For The Original .45-70 Springfield Rifle and Carbine” by J. S. and Pat Wolf" Pat Wolf sells the book and loading dies made to suit the Trapdoor.
http://www.the45-70book.com/
To cheap out and not handload for that gun is false economy. A .45-70 Springfield is not a .45-70 Winchester nor a .45 x 2 1/10" Sharps and will not do its best with ammo made for those and other sporting rifles. Not that common commercial low end .45-70 ammo isn't safe, it is, but it isn't Right and will not do what the gun is really capable of.
There are some good references and even some loading and shooting information at
http://www.trapdoorcollector.com/shooterstext.html
Congratulations on your find.