Dave,
When you are opening to 120, use the brass as a headspace, and when it BARELY closes. STOP .. If your 45/70 rim thickness was cut "generously" you may "see" pressure from mild loads, aka, primer stepping back.

my 550 express is done on a vz24, but the ruger 77mkii would be my preferred action for that. it's WIDE

www.weaponsmith.com and click on 550