Sounds like a fun day!
SKip the 45/70 -- anything that the 70 can do, with proper loading, the 120 can do at seriously lower pressure.

I've tried the slower powders.. and have given up on them.. they burn WAY too much outside the barrel. A little filler is 30 seconds per case more time. no worries

Get some norma brass, and have him headspace off THAT brass. Its a touch thinner than other stuff, but looks tough as nails.

I've got a 45/70 double rifle.. a spartan .. and its very light.. can't even begin to load it, at safe pressure, to even get close to a low pressure 120 load.

seriously, don't waste your time with a 70

jeffe