Overheating!!!!!

[airbrushguy]

My '71 C10 which was a 6cyl automatic until 3 years ago when I put a solid running 305 ci SBC in it has me baffled.
I took it for a substantial two hour ride up the NY Thruway and it overheated after a half hour into the ride.
Last year I put a new aluminum radiator with two electric fans that run all the time, new hoses, thermostat, water pump etc.
The engine is a stock 305 with a mild cam, headers and Edelbrock 4bbl. When I put the v8 in it, I replaced or rebuilt everything except the trans and rear but they were both cleaned, checked, new filter in trans and trans lines and both refilled. 50/50 antifreeze in rad.
The engine would run ok if you did not push it past 50mph, at 60 it does about 3000rpm and that would immediately bury the temp gauge. The engine was truly hot, you could not touch the top hose and when I did use a rag to grab the hose and squeeze it, it was full of pressure.
Truck runs fine around town but was not put to long hi way driving until last weekend.
Any thoughts?

Thanks