https://shop.championcooling.com/Sh...Duster/1971-1972-Plymouth-Duster-SKU-526

That should work fine for your application. I modded a 26" into my Duster but my friend has a 71 Demon with a 318 magnum and one of those, been 4 years already with no issues.

'70 Duster 470hp 340/T56 Magnum/8 3/4 3.23 Sure-Grip