69 Dart cooling questions

Cooling is all about surface area (of the radiator) and airflow (usually the fan at lower vehicle speeds).

I assume you have the two row summit aluminum radiator which has about 1" tubes, so a 2" thick core, yes?

First, since your current radiator is cooling acceptably except for the hottest days, I would try a new fan first. A pusher is not quite as good as a puller type fan. I went through this years back on my 66 Charger bracket car and after trying several inexpensive fans, I learned the lesson and bought a Spal electric puller fan. I put them on all the customers cars now and would not consider anything else. They draw a lot of current, but move a lot of air and that's what the fan is all about!

Your radiator is a little small and if you have the room (or are willing to trim the front core support), I would recommend a large radiator, but the fan would me my first change. Get a puller and mount it and see how it cools. Even if you decide the upgrade the radiator, you will still use the puller fan, so you will not be out any $$$

Another area is sealing the radiator to the core support and the hood to the core support. You do not want any air getting around the front support unless it travels through the radiator! My secret weapon for sealing those spots is garage door lower seal strip material from your local home center. About 1.5" wide, made of rubber and can be easily trimmed to fit.

Hope that helps!