Have you considered bending the existing mount until it is centered over the bore? It appears to be really high above the barrel already, and I don't think any increase in height at this point is going to make much difference.
Perhaps a better alternative would be to take the whole thing to a good gunsmith, tell him what you want, and let him modify the existing mount to lower it (or not), and make it adaptable to take 1" rings. Some cutting, welding/soldering would need to be done, but I'm sure it could be done.
As you are already aware, 3/4" tubes are darn near non-existent anymore. Get that thing(mount) revamped to handle 1" scopes. A 'smith or good machinist could also fabricate an entirely new mount that would work. Just my take on the situation............