Your help with a 6.1 cooling system in a 1971 Duster

That should be more than enough cooling. The stock 6.1 radiator is only single and measures about 450 square inches, while the radiator you're putting in measures about 415 square inches but is three core.

The Champion radiator is supposed to cool 650 HP, you're motor will probably only make 500 HP, so you are well within design parameters. I've seen people run stock 340 radiators which are much smaller with 6.1L Hemis without problem.

Cruising down the freeway isn't a problem, there is so much airflow at 75 MPH that you don't even need a fan. The real concern is stop and go traffic. Given your two fans it should work fine.

However, you might want to run Champions 16" single fan. It moves more air than two 12" fans and is cheaper. More and cheaper what's not to love?