<VV> Overheating

[Jay Botwick]

I have a 65 Corsa Conv. 140hp. 4-Speed.
When on the highway or maintaining high revs around town it overheats.
If I keep the revs low I can drive around all day without a problem.
The only modification is I replaced the exhaust system with headers from Clarks.
Checked the fan belt to make sure it isn't too tight.
Oil is fresh & high zinc 10w30 for classic cars.
Run on 93 Octane Unleaded.
Engine fins are clear and clean.
Filters are fresh.
What is the normal temperature range when warmed up?
Any troubleshooting suggestions?
Thanks!