I understand that the M 70 used a full length action and blocked the mag. box back to '06 length. If that is so, then remove the block or aquire a new mag box. You are looking at a COL of 3.565" +/- to get full performance.
If you long load a 7 Rem. you will get within about 50 fps. with a Rem but with less bullet flexibility.
To rechamber a 7 Rm you will need to turn off 1 thread and then run in the reamer. That's all the chamber takes. Magazine length is the other issue.
Consult an expert gunsmith on M 70's for best advice.
