As an appendum to the above. May I respectfully enquire why you would wish to do this ?. If you have the ability to build this, you can build a .700NE which will do all that the 12GFH will do and more, for the same pressure, and has some resale value afterwards. Same could be said for a 4 bore DR.
Whilst the 12GFH is an interesting achedemic exercise and fun, building a DR is a lot of work and it will only be good for one load anyway because of regulation.
If you have the mechinery you could take a Ruger #1 and scale up the breech block and action to utalise all the other parts and have a gun that would handle any 12GFH load and also be useable with different loads. A modern Farqy of sorts.

I cannot recall the thread used above, but from memory I just choose whatever had a minor diamiter that would suit the ID of the monoblock. I don't have my Zeus up here but from memory it was 7/8 BSF?? Judging from the picture and the length of the mono I guess I threaded about 1".

Regards