Original chamberings in rolling blocks went up to .50-70, although there were very early conversions in .58Roberts centrefire. The action is best suited to "shorter" cases. You'll strike problems with longer cartriges than the .45-70, the hammer height interferes with them chambering. I've seen one built in .45-90 and it was a bugger to feed rounds into, especially with long, heavy bullets.
If you want something a little different than the norm, .44-77 was an original chambering and renowned to be quite accurate. There was a .44-90 also.
My advice would be to chamber it in .45-70. Cheaper, available brass, an original chambering, plenty of reliable reloading information, renowned for long range accuracy and very easy to get shooting that way.
If you build it with about a 30", heavy, octagonal barrel and keep its overall weight to a maximum of 12lbs, you can shoot BPCR silhouette with us. Especially now that you have a 500m range at Bluehills in Tassie.
The above pertains to
black-powder loadings.