Seeing that the model is a 1976 and the fact you have fixed a pinhole in the radiator the problem is likely to be the radiator itself. Only speaking from my own problem similar to yours, if you change the thermostat and it still is to warm to your liking that would be my guess. Not cheap but still better than warped heads and engine problems associated with excess long term temps.

Keith D

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1989 420SEL 105575 miles
Pearl Black/Grey

1990 Chevrolet E-Cab C-1500
293,000 miles

1989 Olds Toranado Trofeo'
132,000 miles