tractorboy,
If there were another plug on the head the accessed the water jacket, and it was the same size or larger than your sending unit, it could be used.
My first choice would be to persuade the counterman at your local parts store to research a new sending unit of the same electrical value that had a smaller pipe diameter.
If all avenues came up dead, a close nipple and a bell reducer, or a 'adapter fitting' would allow you to utilize the existing sending unit in the same hole in the head....maybe.
Stuart
