I need help finding a temp sending unit for my Oliver 550. I ordered one from a very reputable dealer but it will not fit. The one that came off the tractor block has a 3/4 inch male end  and the female end has a 1/2 inch
opening that the sending unit screws into. The new unit is too large to fit the 1/2 inch opening. 
The tractor is a1964 550 Special and I know some question whether a 550 Special was produced.

Thanks for any help.