If you can't get 70 grains in, what makes you think they could?

Seems to me that you are duplicating the original load, assuming the same bullet.

What am I missing?

The original cases were made differently and had greater volume.  

Oh, OK.

Thinner, and balloon head?

Offhand, I would say that you are simply going to have to chronograph them, and run the results through a ballistic calculator.

My Shiloh sharps is in .50-70.  I'm able to use a drop tube and a compression die to get a full 70 gr. in a Starline case with lots of compression and no wad under the bullet.  I've also shot 65 gr. w/ a little less compression. My rifle likes the 70 grain load so that's what I use exclusively now.  I lube the bullet with a 50/50 mix by volume of peanut oil and beeswax.  My load chronographs right at 1250 fps.  The Lyman 515142 bullet theoretically has a BC of .250.  Use JBM Ballistics to run the numbers to get you close.  The problem with long range and black powder trajectories is any small deviation in the data or distance equals a big shift on the target.  Good luck.