I'm with joker, if it only 'overheats' under load, not at idle, I'd suspect it's a timing issue - might also have a loose fan belt, so the water pump isn't turning full speed .... in fact, are you sure you have the right size pulleys on the pump and crank?

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics