26 inch ECP in a 68 Dart

Is this the radiator you're talking about? 26" Small Block Mopar Radiator

If it is, the one concerning thing that I see is how tall it is. It lists the height at 23 1/8". By comparison, the ECP radiator that is actually marketed for A-bodies is 22" tall.

I will say right now I haven't installed an ECP radiator. I have installed a Champion 26" radiator in my Duster, before the Champion CC374 became the CC374B and became too tall to fit without cutting the lower radiator support brace. The radiator I have in my Duster is ~22 3/4" tall, and it pretty much sits on the lower support. The new CC374B is 23.5" tall to the cap, and it will not fit without cutting. The 23 1/8" measurement will be very tight. I'm not sure it will clear without modifying the lower support brace.

As for the radiator size and cooling, it should be more than enough. The Champion I run is a 26" 3 core, and the iron headed 340 in my car pushes 400+ hp. With the Ford Contour electric fan set up I run the car stays cool and consistent even when it's 110* out. Here's the link to my install in my build thread. My car was also originally a /6 car with a 22" core support opening. My "new" '74 Duster- or why I need a project like a hole in the head