Hi again.
I would try that torch thing that PaulS mentioned. Either they used loctite, or they cross-threaded it and ran it on in there with an impact wrench. If they did the Loctite thing, the torch will free that up. If they cross-threaded it, the torch won't hurt anything because you're already pretty much up that certain stenchy creek without a paddle.
One thing that you seem to be unaware of, though, is that you can't just replace the rings like you are trying to. You also have to hone the cylinders. As the rings wear the cylinders from mileage, they leave a step at the top and bottom of there travel. The rings themselves also wear a little bit. When you put the new rings in and start the motor, the new rings are going to hit a step in the cylinder at the top and the bottom of their travel, and that is probably going break some stuff in there. You can hone the block with it still in the car, but it is sooooo much easier to go ahead and snatch the motor out of the car first and have the block out where you can actually work on it, take some accurate measurements, work it a little more, etc... and then be able to clean all of the abrasive crud that you just created out of there. This also calls for having some tools that a lot of folks don't really have, like a dial bore gauge, engine stand, eyc.. If you have any friends that are machinists, then they might have that stuff, but, if not, you'll be needing to do some shopping. Engine stands aren't very expensive, but, a decent dial bore gauge set isn't cheap.
1966 Mustang Crossmember problems 
