A full nitro .500 is going to need to weigh 11 lbs or so to be comfortable to shoot IMHO. Difficult to get this much weight into any converted shotgun, without ruining the handling. Heavy barrels and equal weight added inside the butstock will probably be the gunsmith's solution, which results in an absolute slug of a gun. If you want a lively double, the weight simply must be between the hands.

Have you addressed this in any way?

Sounds like the action may well be up to it, but my recommendation would always be to have the gun regulated for a lighter charge. To me that would mean the 440gr Woodleigh at up to 2050 fps or thereabouts, or the 570gr bullet at perhaps 1850 fps max. The latter would be best if you wish to shoot solids of course.

Either way, you would still have a wonderful killer, and a bigger margin for error pressure-wise. The weight distribution and handling qualities of the finished DR need not be so disasterous then either.

If you are a masochist, and can handle a 9 lb .500 Nitro, then please disregard the above!

What does your No.1 weigh all-up?

Pressure data for the NEs is best obtained from Wrightie's book, which also contains historical tables as appendices. I'm away from my library right now, but someone will chime in I'm sure.

Good luck with the project, keep us posted on progress, and a series of photos of the shottie in question as it meets the angle-grinder would be interesting to see as well.