Squealing going uphill

[cci]

I've had a '70 C/20 (350) for a few months and overall it runs like a top. Recently I've noticed a squealing noise - similar to a slipping belt - when I am on the gas going uphill.

I live in a higher altitude area, and descend about 500' or so heading into town. On that leg of the trip the truck sounds normal. On my return trip though, I can hear the squealing pretty loudly as I'm climbing back up. If I let off the gas, the squealing stops - but of course then I lose speed.

The belts (my first guess) look perfectly fine and are good and tight. The previous owner had the fan clutch replaced, so it looks all nice shiny sitting there. Of course I realize that shiny doesn't equal working properly.

I just don't understand the relationship between the noise and going uphill.

Thanks!