Just checked the Hodgdon annual mannual - with longshot and shot weighing 1 1/4oz probably close to your hard ball - 34gr. coul;d be improved upon.

Of course, wad column plays a big part and one must be careful.

I see listings for 1 1/4oz shot at 1,500fps using 35.7gr. in a Cheddite hull at 11,000PSI.

I've always thought shot produced more pressure than a ball, this being due to the friction of the shot mass against the bore's wall in passing up the tube, than does a single round ball barely touching or riding on a gas check as I loaded them. I am therefore of the mind that the longshot load could be increased carefully, watching for normal shotshell pressure signs - primers & stickiness normally.
Sealing the end of the case is a presure reducer compared to a crimp, so that itself leaves room for experimenting. You wouldn't want to change too much, as the grouping is so good - but - it might be necessary to put up with a lousy 3" overall group if they shot together.

I've purchased a .710" mould from Jeff Tanner, UK for testing in my rifled 12 bore barrel, but haven't tried it yet. In WW, they weigh roughly 500gr.

I'm told 1,700fps is easily accomplished.

Have you tried a lighter projectile9smaller ball using gas-checks to keep it centered - or the Lyman 14 bore 506gr. (WW) bullet in a red wad?