hi flow fans are used alot in applictions where ambiant heat is a problem, also in cotton mills etc.
its basically just a smaller pulley to make fan turn at a faster ratio for more air flow. Sometimes you get a special fan to accomodate for higher rpm rate.

Some things to ponder:

*is fan on correctly? (hopefully not on backwards)
*does hood need extra venting for better air flow?
*how does truck run with hood open? still run hot?
*if radiator is painted, remove the paint, paint acts like an
insulator and can make one run hotter than normal.
*hopefully you have blow truck off and its free from dirt and
dust (especially the engine and radiator area)

*you say a paperroll clamp? how does the truck run when you just drive it around without operating the clamp?

*are you running the correct mixture of coolant to water ratio?
a 50/50 mix is a must, DO NOT run pure coolant in the system.
*have you tried running without a thermostat to see how it
performs? does it still run hot? remove the thermostat, throw
it in a pot of boiling water and see if its opening. another way
to check is watch the water in the radiator and see if its
flowing when the engine is running. Generally when the
thermostat opens and closes you will get a rise and fall of the
coolant level. (ok you already did this so disreguard)

Of course BE CAREFUL because if it runs hot it
will run over and spew out everywhere when you least expect
it. Coolant burns are very painful and can be bad.

These are just a few things that come to mind while pondering your overheating issue. Good luck!