Factory Y blocks do not use a temp sensor on the passenger side, only on the driver's side where the threaded hole is.  You can re-thread the existing hole, or buy a new bushing from one of the part suppliers like Carpenter or Concours, or Macs, etc.  The early cars had a 3/8 pipe thread, after '57 they went to 1/4 pipe.

John - "The Hoosier Hurricane"

Hello...I want to install OEM 6V temperature sensors in my '57 292 heads and get the dash temp guage to work. The car came with a freeze plug in the passenger side hole where the temp sensor is supposed to go. The driver side has a threaded insert installed for an aftermarket sensor that is too small to accept the OEM sensor.

I've not done this before. Can I buy threaded inserts that press into the freeze plug holes...or do I need to thread the existing hole in the passenger side and drill-out & thread the hole on the driver's side? Thank you for your response in advance...Bob