I remember that post. Beautiful work Transvaal.
No doubt that a proper gun can be built on a larger frame. The question is whether the owner wants to balance more like a rifle or a shotgun or how much weight you want depending on the load you are using. Nobody likes to get beaten up.

Longknife, since you live in Illinois, take a look at Simpson Ltd. They import a lot of firearms and have some great platforms.
If you want hammers, they typically have Husqvarna 51s and some Belgian guns for reasonable numbers. Those models are relatively modern guns with Greener crossbolts which is a plus.
You didn't mention the level of machine equipment you have access to.
Less machining to sleeve a 20ga barrel into a 12ga monoblock though.

Transvaal, did you ever build that small bore double rifle on the AYA?
Steve, a short case double 58 sounds like a great rig.

Tom