Proof loads? Why bother? You have already exceeded the design pressures for the action you have chosen. SAAMI Maximum pressures for the 12 gauge are 11,500psi and for the .444 Marlin they are 42,000psi. You may have put some stronger barrels in the gun but the action is still a shotgun action. Here is something I posted on another site regarding what you are doing:

"I had a conversation with Paul Jaeger several years ago about making a .375 O/U on a Ruger shotgun. He had done some conversions like that before and chose the Ruger because of the strength of the action. He said he was no longer converting them because of "problems". I asked what the problems were and he went into a discussion centered around the weaknesses of a shotgun action and problems of overworking it. He mentioned double firing. He mentioned setback. He mentioned safety. A well known USA double rifle manufacturer also gained some early notoriety by converting Ruger shotguns to double rifles. You can sometimes find those early guns for sale online, most often in in .444 Marlin or .375H&H. These days, they refuse to do a shotgun conversion - for good reason.

But bold individuals continue to make cheap conversions using ... shotguns as the basis. I wish them good luck and hope they continue to keep all their fingers. After all, anything works until it breaks."

I would think an engineer would have a good grasp of what's going on. I say, if you really must, take it out and shoot it till it either scares you or blows up.

Good luck.

Grenadier