SLT-6 is 0.060 Over - Runs Hot...Why?????

hi and hello:

I think I have a problem. My 1968 Dodge Dart 270 2DR HT with SLT-6/225 is running hot after 15-20 mins. Temp gauge starts at mid-point (normal) then goes right to RED ZONE.Temp will come down to mid-scale after I switch on heater. Temp might go up again depending on my driving. For the most part I am running very near to the RED ZONE when driving. Don't like this fluctuation and high-temp so I know I have a problem. Car has less than 1500 miles after rebuild and sat for almost 25 years because of corporate moves, no interest, and life's events. Totally restored, pretty, and a real charm but motor is the issue.

Motor was rebuilt in 1995 and builder said bored to 0.060 (should have said STOP and I need a new block). OEM rad re-cored, new 160 deg stat, and new OEM water pump. Timing -5 deg and dwell 43 deg. 50/50 anti-freeze installed, new sender. Motor fired up in 2020 (finally) with lots of pre-lubrication, runs very smooth I must say on the highway.

Question: is 0.060 too much so that minimum block mass to extract heat? An old MOPAR guy told me this is my problem so try a bigger radiator. Alternative is re-sleeve block. Motor is original from car and afraid to try a SL-6 after 1970.

Will tear into this in April after vacation in FL.

Thx for your thoughts and comments. I love A-Body Forum. Great experience and lots of help.

Love my Dart...