With chamber pressures that the developer of the cartridge you have chosen "Thinks" it will develop you need an action with firing pins discs. The Brno zp 47 / 49 actions built in the late 1950 and early 1960 are very well suited. It is made of through hardened high quality alloy steel.

You can view some of the work on a double rifle I built using a zp 49 action.

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=297834&page=0&fpart=1&vc=1

Search this forum for posts by Ron Vella of Canada who also used the zp 49 actions for such a project.

In regards to books that might recommend threading barrels to screw into the mono-bloc of a set of shotgun barrels: it is a poor idea not wise as it decreases strength. Also adding firing pin discs to an action made without them weakens the action. Actions made without firing pin discs originally have considerable recesses milled in the back of the action to have room for the hammers with firing pin nose/pins. Therefore drilling away the face of the action even further removes metal and causes weakness.

Here is one of Ron Vella's many posts to this forum:
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=185191&page=0&fpart=1&vc=1

Your background, training and shop equipment appear sound to learn to do what you will have to do. However, the regulating of the barrels to shoot to a certain grouping at a designated range will be the issue you will have the most learning. As such I do want to comment on the cartridge selection you are proceeding with. Shortening a .45-70 cartridge is counter intuitive in double rifle building as the longer standard .45-70 cartridge can be loaded to many configurations and having room for enough propellant in the cartridge case is very useful in regulating the barrels in the finish double rifle. Regulating barrels is a task not to be taken without considerable thought and that thought begins before you make the first mechanical action in your workshop--the decision on how much convergence do you build into he muzzles of the barrels for the velocity of the projectile that you have selected. Regulating barrels on a double rifle can be a done in a few hours or can be two monthsif the cartridge is not suitable.

It appears that maybe if your cartridge will not regulate you can lengthen the chambers to standard .45-70.

Stephen Howell