savage,

I'll provide what is pretty standard advice with these conversions.

Get hold of Ellis Brown's book "Building Double Rifles on Shotgun Actions".

It will answer alot of your questions.

Yes this is shameless promotion of the book.......but I wouldn't do it if I didn't think it was a great book. I'm earning no money from promoting the book.

There are calculations in the book to determine the stresses applicable to actions, generically speaking, that will help you determine appropriate cartridges for your actions.