If the shot cup has a well cupped hemispherical shaped gas check base, cut that off and place on the powder. Then use 9 or 10 bore, fiber wads (whatever is needed) to fill the case to the ball. Place another gas check cup under the ball with the cup holding the ball centred in the case and in the bore - I'd start with the .748" balls.

In my testing, I did not have good luck using compressible wad columns under the balls.

That said, using Blue Dot or Long Shot powder, I would be surprised if you didn't see 1,900fps with a .690" ball.

That would put your 10 bore at a higher speed than any bore rifle, but a good 200fps.

It would take VERY careful loading using a chronograph and any other pressure judging methods possible to stay safe. Perhaps you should think in terms of staying in the 1,600 to 1,700fps range.

1,650 to 1,700fps would be equivalent to the 3" 9 dram heavy 12 bore charge of the 1890's, early 1900's. Peter from Denmark, of this forum, has just such a gun, I believe.

To find out what speed to develop your loads to, use a 10 bore black powder load and chronograph it. W.W. Greener's 9th Edition book should have that data, or perhaps COTW or other tomb.