I think the only way to put those barrels back in service would be to have an action custom made for them. It simply would be an incredible coincidence to find an action to which they would just mate up.

You are fretting over what gauge to seek out, but the critical dimension for first consideration is the spacing between the firing pins. This must precisely match the distance between the centers of the bores on your barrels. Start your search there, and maybe you'll get lucky.

Good luck,
Curl