Hi Canyonman and welcome to the forum.

Modifying the action to incorporate a rib extension and crossbolt is a pretty big job, especially for a first DR project.
There is a lot of work involved, and it would need to be done very accurately for it to provide any real benefit, regarding the overall strength of the action.

A good place to start concerning how well a shotgun action will stand up to rifle pressures, is to do a breech thrust calculation for the original shotgun cartridge, and the same for the rifle cartridge, and compare the two.

This will give you a "rough" idea on how much extra strain, (if any) could be imposed.

I just did a very quick calculation, and the 12gauge, based on a 12,000 psi working chamber pressure shows approx 6,000 pounds force against the breech.

The .450NE 3.25" at 44,000 psi chamber pressure shows approx 10,000 pounds force against the breech.

It gives a bit of an idea on the importance of ensuring everything is done right during any shotgun to DR conversion, as the stresses on the action can end up higher than what the original shotshells produced.

From memory Butch Searcy was supposed to have fitted a rib extension to at least some of the BSS conversions that he did way back when? (I read this somwhere on the net, so do not know if it's correct.)
If he did so, you'd think there would have to be a reason for that extra work.
Hope this helps.