Ford F-150 - 1997-2003 :: 2000 - Overheating Above 55 Mph - Replace Engine?

I own a 2000 F150 4.2L (I think off the top of my head) v6 XL model. For the past couple of years I've had an issue with overheating. So far I've had the following replaced:

Radiator
Thermostat
Radiator Cap
Water Pump

The intake manifold has been repaired, that was the first item to have some work done to it. The shop I took it to said the coolant hose that runs into the manifold was corroded at the entrance to the manifold. They stated there was no way to actually replace the hose, but they gave me two options:

1. replace the entire manifold or
2. cap the existing entrance port, drill a hole into the manifold next to the port, and run a new hose.

I chose the cheaper option which I suspect is part of the problem. Now to the problem.

I can drive around town all day long no problems even in 100+ degree Oklahoma summer heat. I left it idling on the side of a street for 2 hours during summer and no problems. The moment I get the truck to 55mph it starts to overheat. Not just overheat, but peg completely out in just over a mile after hitting 55mph.

Once I drop the truck down to about 40mph the temp starts to lower, but will fluctuate between normal and 3/4 to max.

What I need to look at? I've been told head gaskets, but there's no leaks and no water in the oil. I've been told to replace the engine, which if I could afford that I could afford a down payment on a new truck. It's a bit frustrating at best.