Holly
Does your "some building experience" include some years, thousands of hours, building and reconstruction of this type of structure? Regardless, have you talked with and had somebody who does this kind of work (on this type of building) look this specific case? (Always best to talk with several such folks, of
course. )
I don't do really old stone buildings. But my experience in
wood construction (besides making my living in a past life as a residential plumber in San Francisco, I have maintained and rebuilt two 100 yr old building for the last 20 years) has been that we can all understand how each part of a building works, at least after some study, but putting the thousands of details together to fully understand the "situation" in the field requires considerable experience. And to give you the best choice means an depth knowledge of appropriate and current building technology available in your area.
I _have_ studied seismic construction for the San Francisco area and retrofitted one 80 year old building. Seismic building science takes some real study to get the basics and some real current experience to design a cost effective approach. To be effective it must be designed as a _system_ which includes many different parts of the building. I am not trying to discourage
plans to make any parts you add strong, but I am noting that one strong piece does not make a building strong seismically.
You are dealing with the basic structure of the building, what holds it all up and together. The consequences of missing something important there can be extreme. So I am raising a flag here. If you do not already _know_ how to build that type of building advice from strangers on the internet is not something you can evaluate well.
Please forgive me if you are in fact experienced experts in this field. I just don't want innocent people to carry on thinking free advice on the internet will provide them the knowledge and skill that could mean the difference between a warm happy home and the house falling down. Or any of the thousand (expensive) alternatives in between. My own experience, over and over, has been that one _must_ see the building in person to hope to make any helpful recommendation AND that experience, a _lot_ of experience, makes a world of difference.
Regards,
Rufus