Im in no position to talk technicalities, but when I owned my '50 shoebox, I had the same problem with the Flatty (Stock 239ci, non-OD) running hot. I would run it around 70mph on the highway, and the temp would climb to the 200's. However, 3 days after getting the flatty back on the road after 30 years, before starting the 400-mile (round) trip to Billetproof, I filled er up with some Marvel Mystery Oil. I was running a VERY STEADY 180 degrees going 75-80mph on the highway the entire day. Keep in mind that was without and OD tranny also. People thought I was Nuckin' Futz, but I didnt have ANY problems that entire trip. I wouldn't hesitate to do it again....If I still owned the car.