Hi Jeff

Well the long and short answer is that whatever you do it is an apples and oranges thing. The 13 tons (approx) of the 450#2 is a base thrust measurment using a base copper crusher and is not really directly comperable to any other measurment to a degree that can be exactly proven.
I would take the whole rim dia for the calculation so that you get a worst case scenario
Hoop pressure/stress is not going to be an issue if you are using commercially availible 4140 (En19/24T) barrels. What is is breech thrust. Now if the 10 G has 11mm + wide bites and a decent length of watertable then I would think that you will be fine (that is not a warrentee or anything like it for the litigious.
What is likley to happen IMHO is that it may come off the face for the proof loads and need to be re-hooked. This is not because it is in danger of imminent failure, rather that when building a new DR it is made super tight (ie you have to force it closed) this is to allow things to bed in for the first few shots it is then eased off in finishing. Now this gun you have will already be bedded in and may move a couple of thou off face. This can be easily corrected if it happens (I think it will) by going to a shop that does industrial hard chrome plating and have a few thou or whatever is needed plated onto the hook and then stone her back in (you need diamond stones/files for that incidentaly it is super hard)
Obviously things have come a long long way since I did that conversion. I would not if I were in your position use the old monoblock and sleeve. It is just as cheep to make a set of dovetail lump barrels, which will give a better finished job, allow you to re-file the detonating and standing breach (as it is certainly too big and clumbsy in a 10 G) and it will make a far better finished job for the same imput for you and cost.
If you want a JPG of a dovetail set up to help you on your way then drop me a PM and I will take a JPG off of the CNC program for you. It is a $ 200 for a machine shop to make the parts from 2 profiled barrels and a $20 peice of 4140. I would imagine it would cost you about $ 50 to get brazed up by a specialist.
It is much more satifactory and you will be hooking it correctly from the get go.

Regards