CIP standard for 577 snider is 1:20. So 1:70 is not going to cut it, that is a ball twist realy.

$ 645 for 2 barrels is very good. especially if they are going to turn the OD for you. Even if you machinist is set up to do this you will pay him at least 4 hours labour to do this.

I would order the 1;20 and spend the waiting time getting everything else prepared.

Regards