I have a 69' 3 door Chevy burb. I just put serpetine drive, a new reverse flow water pump, new performer intake/gaskets etc, flex fan, and temp sending unit. The serpetine drive came out of a 1990 350 from a chevy suburban. I have changed the themastat twice and it is still running hot on the hwy. Idling in the drive way is no problem...it doesn't run hot until you get on the hwy. I'm not sure why...Any thoughts??