I would like to do a .750 single trunnion pressed through the barrel.
If you go all the way through, then your depth will be limited by the placement of the bore for the trunnion.
Typically, about two calibers is the common depth for a mortar bore. In other words, if you drop two balls in the bore, the second one will be about flush with the muzzle. Then you will need to add the chamber.
You will want to have at minimum, a thickness of metal equal to the diameter of the chamber between the bottom of the chamber and the hole for the trunnion.
As you can see, it adds up fast. But if your tube is long enough, then no problem.
Most mortars with side trunnions that I've seen here had trunnions that were either screwed or pressed into sockets from either side, then welded. They went in only deep enough for stability and not so deep that they would interfere with the bore and/or chamber.
A more common arrangement is as you would see on machined replica Coehorns, where the trunnion fits into a recess milled across the end of the tube, and is then brazed, bolted, or welded in place.
Of course, on the originals it was all cast as a single piece so trunnion placement didn't present these problems.
