Radiator core tubes....

On the stock 22" 3 row radiators for the 70-72 340 Dusters, what size were the core tubes? My 71 came with a 4 row when I bought it, but has always has trouble keeping it's cool. Ive flushed the thing, boiled it, etc, but it always runs on the hot side in warm weather. Out of clear random, I happened to notice the core tubes are only 5/16", so that's only 1.25" of cooling area for one row of tubes. By comparison, a '70 Cougar XR7 I have been working on at work has three rows of 9/16" tubes for 1.75" for one row. Thing has stayed cool in 100 degree days.

I think this 4 core isn't all that it's fluffed up to be.