Would my Duster run cooler with an electric puller fan?

Exactly, just what I was going to state.
I'd like to ad that a good clear clean flowing radiator with a fan shroud will work excellent. I would couple it to a viscous clutch and fan. Coolent at 50/50 min. mix. Hopefully the water pumps impeller isn't rotted away as well.

While your under the hood, a quick check of the cars fuel ratio would help and is cheap. (Plug reading) Plugs are cheap and rejecting of the carb is quick and simple as well as cheap.
A basic tune up if need be. Works wonders vs. the untuned state, since we're talking about a new to you car. A poor running engine could drive up temps easy.