Greetings Longknife.

Stick with the smaller bore gun. Go with a gun that has better lockup as well.

When I built one, I went with a 16ga gun and used 20ga tubes and monoblocked. That project worked out fairly well but as stated earlier, it is really easy to go overweight when using a larger frame.
The math I used was that the wall thickness needed to be at least .090 so in the monoblock area it was easier to keep my barrel blank to that number when it was sleeved into the 16ga block. Go with solder. Shotguns are easy to work with.
I am presuming you are going with rifled blanks.

Whatever blank you go with has to have a groove diameter same or smaller than the inside diameter of the shotshell hull. In my case, Brenekee hulls were.625 on the inside (IIRC) so that was the groove diameter, and I went with .005 rifling (IIRC) with a slow twist. Shot fantastically.
The negative is that it had to be a custom barrel.
Factory smoothbores are up around .620

If you can use brass cases (which will also give you more capacity), they are much thinner and you may be able to get away with a repurposed factory barrel and simply send it out to be rifled.

100 gr is a good number.

Cheers,

Tom