IMO I think you should check the radiator too, it might be blocked internally or some of the fins have rotted away decreasing the radiator's ability to disipate heat. Because at those speeds and above there should be plenty of air to keep the temps under control.
You can check the fans manually, the plug for the radiator fan is under the battery. Just use test wires to hook it up to 12v. The temp sensor for the fan is located on the back of the block below the intake manifold.
The fan relay is on the passenger side near the headlight. To check the fan relay (it should be clip side up) there should be continuity between the upper left (a) and lower left (b) connections when 12v is applied to the upper right (c) and lower right (d) connections. GL
Code:
Fan Relay
____---____
| a__ c__ |
| b__ d__ |
__________