I can almost promise you that it will leak. If you have installed the kind of sending unit I think you have, then it will have been an 1/8-27 NPT or metric equivalent . This is obviously a taper seal and will not seal with only a few threads and the pressures at hand. My recommendation to you, is hit up the local parts store and get a 1/8-27 plug, then re-tap your thermostat housing closer to the block; preferably at an angle so that it doesn't disrupt the flow too much. If it does seal, and again if it DOES, you still run the risk of interfering with the thermostat's travel; so why risk it?
just my
