I've just gone through this exercise on a 1976 911 rebuild. After days of heat/blaster/tapping with no discernible result, I made peace with the fact that I'll have to replace the thermostat ($359 on ebay). The reason being that splitting the nuts on the brass lines was not an option, as that would require replacement ($1200) of the brass lines. (Unless you had some replacement nuts lying around and be OK with cutting/soldering the lines).
The pipe wrench and 6' pipe extension in the bench vise got the nuts to move by shearing off the aluminum threads. Removing the aluminum from the threads in the nuts was accomplished by heating nut with mapp/oxygen, and digging out with a sharp awl. Be careful to insure it is all removed, as not to wreck threads on shiny new thermostat.
