ms,
I hear and feel for you! I have RA myself and it sure doesn't help the shoulders. The mercury recoil cartridge is usually fitted in the rear stock by drilling out the mountingbolt hole somewhat. Perhaps if you do an on-line search for the manufacturer you can ask them what's involved in your installation. I bought a Past Decelerator Pad from Cabela's that is quite inexpensive for range use and they come in standard as well as magnum padding styles. Before you have to spend allot on modifications on the rifle, try the Past system or a good slip-on recoil pad.
Jim