Get a Robert Shaw design (Flow Kooler, Milodon, or CDP unit off Ebay) and try that before chasing ghosts with the Rad. Stay away from the Mr.Gasket unit as they seem to fail fast per Big Brother Google searches.
Personal experience: Have a 4.10 geared car that stayed cool-ish around town with a US Radiator 3 core High Efficiency 26", but when going over 2500RPM on a long drive it started to creep up to 200+ and stay there. Using a high flow pump, Clutch fan, 7 blade OEM, shroud and all and it would not stay cool at speed. I was too cheap to spend $30 on a T-Stat and thought a T-Stat was a T-Stat....wrong!!!! I figured $30 was a last ditch effort before spending $1000. I put in a Milodon 180 and it has corrected every issue I was having for the last several years.
I had a parts store 180* thermostat in my 69 Dart 383. It would run cool until I got to higher speeds, then it would get hot and there was no cooling it down. I had a brand new Cold Case radiator, so I bought a high flow Robert Shaw thermostat, and did a quick home flush of the radiator. Now the car will run 180-195* all day long (except sitting in traffic, but that is because I don't have a shroud yet).
