Thinking of installing a flex fan, same size fan as stock, on my '71 500. Will it provide enough cooling or should I still with the stock setup?

If it ain't broke...

It shouldn't cause any problems, but then again, it shouldn't give you all that much of a gain, either.

If you have a fan clutch on your current set up that will also have to be removed and a spacer installed. A flex fan will not cause enough drag to engage the fan clutch. I would stick with the stock setup. In my 69 Caddy hearse (472) when I bought it there was a flex fan setup. I found the original seven bladed fixed steel fan in a junk yard in town and installed it with a new fan clutch. I figured if it was engineered that way by Caddy it was good enough for me.