The 45-70 is a real deer killer and cast bullets can drop them as fast as any premium bullet. BUT I'd stay away from the cowboy loads. I'm sure they're fine loads for compitition shooting but just not for deer hunting. My advice to you is to stick to real hunting loads designed to kill deer. They may kick a bit harder but the distance you're going to drag that deer back to the truck will be a lot less.
Try loads from http://www.buffalobore.com/