260rem. your load of up to 70gr. is easily OK, likely up to 100gr. or more, as well.

The velocity will be very low and I think you would be better off, most likely with anywhere up to 7 drams and one ball, however, that big load is hard on stocks.

I would find a load with a single ball that shoots under 6" groups at 50 yards.
A realistic max. load would be in the 125 to 150gr. area, 150gr. being in the 4 1/2 to 5 1/2 dram arena.