I drove my hot rod around today(48' 59a in a model a) 70 or80 miles and at 75 degrees out it only reaches 150 degrees at 60 mph when i stop it goes to around 140. i checked my guage with one stuck in the radiator and its within a few degrees. what can i do to get the temp. up? do they make thermostats that go inside the rad hoses? or ive heard of running restricters with a 5/8 hole will this help?