If you have enough cases to have 2 sets, 1 set for each gun,
then cut down some of the 45/120 brass to 45/70.

The only money you are saving is in the cost of the Brass
and using your exisiting 45/120 brass.

But how much will a re chamber cost ???

Save on the cost of the re chamber and buy some Brass with the same money.

I really think you are barking up the wrong tree
and it is all false economy.