Imho, an auxiliary fan will help but the hot air needs to be vented, otherwise, the fan will just be moving around hot air. A combination of a fan and vents will work much better. The vents can be a late 280 hood or cutting louvers in the side access panels by the hood.
Another idea for consideration is a flex fan. The vast majority of our members think flex fans suck, and that may be polite. My experience with one was good. I installed one on my 71 when the OEM fan clutch seized. One of the first things I noticed was it took longer for the temp. gauge to rise in traffic, which told me the fan was pulling more air at idle than my original metal seven bladed fan. I also liked the way the blades flattened out around 2000-2500 rpms. My original plan was to go back to the OEM fan set-up but the flex fan worked so well that I ran it for another 20 years. One of the things that still bothers me about the OEM fan clutch is everyone that I've ever spun by hand felt different. The resistance was never the same.