Liquid filled thermocouple relocation

[SofaKing]

I replaced the 59A with a 36 21 stud in my 39 pu. The thermocouple tube is too short to reach the water pump fitting on the front of the 21 stud engine. My plan is to weld a boss onto the steel tube from the pump to the radiator. I thought the nut on the thermocouple was 3/8 pipe thread but an iron pipe coupler did not seat. Is anyone aware of an off the shelf fitting that will fit the thermocouple and seal on the flare?