Alternately, you can go to the 45-70 classic: Accurate 5744
A5744 is one powder that does not object to low fill ratios.
I suggest starting at 24.5gr under the 539 cast bullet you mentioned for a low 1,300-ish velocity (34" barrel again)
which will have
very low velocity spread.
See:
http://www.accuratepowder.com/wp-con...d_data_3.5.pdf