Cooling Issue - 1964 Dart GT - 273 with automatic and A/C

The factory A/C car got a 3 row cored radiator

That is not the case. For one thing, there was nothing such as a factory A/C A-body in '64; that started in '65. The '64 A-body A/C was still a dealer-installed package, which did not include a new radiator. And even in '65, the factory A/C cars didn't get 3-row radiators.

I don't believe they had a shroud that year on the V8.

Fan shrouds were available for the '64 A-bodies with \6 and with V8 engines. They were typically installed on cars with A/C installed and/or trailer-tow equipment. '64 was the last year for the ring-style shroud; starting in '65 they were box-type shrouds. Both kinds were 2-piece items, upper/lower.

Screen Shot 2024-08-20 at 14.35.20.jpg

I would use a rigid 6 or 7 blade fan blade

Read this.

mostly an air flow issue at idle/stop.

Yup, low-speed/stopped-in-traffic heatup is usually a matter of insufficient effective airflow across the rad. Make sure there's an adequate fan, a shroud will help quite a lot, and install the hood-to-rad-support seal on the underside of the hood; that's another A/C and trailer-tow piece to prevent air going up over the radiator instead of through it. Gary Goers № W81, available (if anywhere) from Quirey Quality Design, the outfit who bought out Goers when his health failed.

The '64 A-bodies with a V8 had one strike against them with that small radiator

As built yep, they did. That strike could easily be erased even back in the day by having a 3-row core put into the stock rad (easy on the V8 cars; plenty of room between front of engine and back of rad). Decades later, high-efficiency rad cores with denser fins made it even less necessary to try to hack in a wider rad, and now we can do it even better still.