The .45-70 was originally named the .45-70-500 Government. This meant it was a .45cal bullet (.458"), 70gr of black powder under a 500gr hardcast lead round nose bullet. If it worked 125+ years ago, it'll work today, too. And yes, you can compress the charge that much. The more the better with black powder...just do NOT slam the bullet home. Just use enough smooth pressure to seat it. Remember, black powder is impact sensitive.

I met a guy that didn't even bother to measure the charges. He had a hand-held press and was loading as he went to feed a Sharps replica. He just filled the case up level with the top with Goex and seated the home cast lead bullet (looked like about 300-350gr, NOT the big 500gr pills). He was getting what I would call very respectable accuracy at 200 yards with that rifle. Looked like about 4-5" group with the vernier-type iron sights.

Black powder is a low-energy propellant (relative to smokeless powder, that is), so I doubt there is any significant risk of overloading it because the case simply won't hold enough powder to produce dangerous pressures.

Mike