Just my opinion (and well worth what you get to pay for it) - I'd leave the buttstock alone and put on a strap on the shoulder pad if you wince that much with your hot loads . . . or just tone down the loads a bit (not trying to be a SA, just seeing those as viable options). No real reason to shoot hot loads from a 45-70 - in all likelyhood the bullet will hit the ground before it reaches the 200 yard mark (unless you scoped it) and under 200 yds, a bullet traveling at "normal" speed loads will knock down whatever you are shooting at.
Do as you are led to do, but I would leave it alone. (I have and shoot 3 different 45-70s and a 45-90, one of the 45-70s is a Marlin 1895 CB.)
Good luck.
