Try brute force.  Get a pipe wrench with a cheater pipe to fit on the end.
Then use the 300 or so ft/lbs of torque to twist it off.  Or keep
hammering away with a hammer and chisel until there is very little left.
EVENTUALLY  the rust will give way and you'll get it out.  This is not a
very strong piece of metal and it can be BEATEN !!!

James (the brute)
'67 deluxe
'71,81,'85 Westies

>> Anyway, my new problem is that I cannot get the O2 sensor out of my
>> cat. conv. it has rusted into place.    I have sprayed the bee-jesus out
>>of >>it with Liquid Wrench but it still will not move.

'67 deluxe
'71,81,'85 Westies