Older flex fans were very prone to cracking and flying apart. When I first put my posi in my 74 years ago I went to do a burnout and all of a sudden BOOM and the truck started shaking like CRAZY! I quickly shut it off and opened the hood. The fan had sent a blade through the schroud, through both layers of the steel hood and the blade landed in the middle of main street about 150 feet away. Prior to that I was under the hood reving it up. Luckily it didn't take my head off.
Flex fans are much better quality now and work very well but can be noisy. They provide less drag which you will notice right away if you have a heavy clutch fan hanging on the front of your water pump now. The downside like I said is they can be a lot noisier than your clutch fan. Don't ever use an old flex fan and if you've had one on your truck for a few years, periodically check it for loose blades or cracks around the rivets.
On my 74 for now I have a flexfan until I decide exactly what I'm going to do for an electric fan... I drew up a schematic for someone on here a while back to give you an idea of how easy it is to wire one up.
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