When it comes to radiators, there have been huge advancements over the years. From down-flow copper brass radiators to aluminum cross-flow to hybrid radiators made from aluminum and plastic, the radiator is arguably the second most critical cooling system component behind the water pump. The most important factor when considering a radiator for your Mustang is the surface area of the radiator core. Maximize your surface area for the best cooling efficiency. If your core size is smaller than your radiator support opening, you're not taking advantage of the available air flow opening/surface area. Once you've maximized the surface area of the core size, you can then go thicker with the radiator core to further increase cooling. Adding core thickness does not increase the radiator's efficiency as much as maximizing the core's surface area however. A thicker radiator core does have more resistance to airflow than thinner radiator cores, but the difference is typically minimal considering a 4-inch thick radiator core has roughly 10 percent more resistance to airflow than a 2-inch radiator core.
Core thickness has been blamed for decreased cooling many times, with the thought being the air flow through the thicker core was reduced and became fully heat saturated before exiting the core. Sounds great in theory, but usually what is happening is a decrease in coolant flow, not air flow. An older down-flow radiator design like that found in a vintage Mustang usually has a very narrow cooling tube design, which is fine with a stock water pump. When you move to a modern aluminum radiator with wider coolant tubes, there isn't sufficient coolant flow to create turbulence in the tubes with a stock type water pump. You need this turbulence to force the coolant against the outside walls of the coolant tubes for thermal transfer to the tube wall and subsequently to the air passing over these coolant tubes. With the combination of a modern large tube radiator and a stock-flow water pump, the velocity decreases and you don't get the turbulence you need for proper heat transfer. If the radiator core is doubled in thickness, the coolant velocity is halved. This is why we recommend a modern cross-flow design in vintage Mustangs whenever possible (short of a concours car that needs a stock radiator), as it uses wide tubes with less cross-section (thinner cores), which requires less velocity to achieve the needed heat transfer.
One thing many people don't consider when choosing a radiator is fin count. Yes, we want the largest core possible, but the fins between the cooling tubes of the core are what give the radiator surface area and help transfer the heat to the air passing through the core. The higher the fin count, the better the radiator will cool compared to a core with fewer fins. The main concern with higher fin counts is keeping the core clean of dirt and debris, as the higher fin count traps these insulators more easily, decreasing the efficiency of the radiator. Fin count doesn't come into play much unless you having a custom radiator built to your specs for a modified car or custom project but it is something to consider if the radiator manufacturer offers optional core/fin count options.
The last thing to consider when it comes to radiators for vintage Mustangs is whether to retain the original classic style down-flow radiator with tanks on the top and bottom, or to update to a more modern cross-flow radiator. If there's room for a proper cross-flow core design, and the Mustang doesn't need to have correct under hood appearance, we recommend the cross-flow design for the reasons mentioned previously, plus they position the radiator cap on the low pressure side of the system. A high flow pump can create high system pressures, which will force water/coolant past the radiator cap at high rpm, something that's terribly common on down-flow radiators, which is why we also recommend the highest cap rating you can find if you're going to keep your standard down-flow radiator due to space constraints (the stock 13 psi cap doesn't cut it with today's engine rpm capabilities). For late-model owners, a cross-flow radiator is standard in these applications so your main area of concern should be core thickness and fin count.