I don't see a problem with the ball either. When I subbed, I was also using a ball lighter than the shot charge.
The shot loads I used, were 1,340fps loads and I easily got them to 1,550fps with smokeless powder - which duplicated the 7 dram black powder load for speed.
Greener's book will tell you what the BP load was, when you get your brass cases.
Made me curious - the only 10 bore load I see at a glance, is:

.775" - 670gr. "bullet" = slghtly hardened Round ball as a .775" ball's weight in pure lead is listed as 700gr.
273gr. powder, = 10 drams
11 pound "rifle"
1,600fps = 3,829fpe
recoil is listed as a mere 79.8pounds sounds like fun so far.
I expect a smokeless load at same speed with a 1 1/4ounce ball will have recoil in the 35 to 40 pound region- quite managable - but, one should fire the BP load, just to experience it. Make certain all your stock attachment screws are tight. BP kicks more than smokeless.