Radiator that will fit my 64 barracuda for a 340 upgrade

The fins per inch and the tube ID are two of the most important aspects of a radiator.

ALL heat transfer happens at the junction of the fins to the tube (sure you get a little from the surface of the tube, but nothing like the fin to tube contact patch)
More fins = more contact + more surface area for heat to be transfered to the air.

Usradiator.com has a great video describing radiator tech.

Lastly... Search FABO for "overheating"

Note the first few lines in most of the posts...

Aluminum radiator, electric fans!

I'm just saying!