The five and six blade fans work off the same pulleys as the four blade fans.
GM offered a six blade dealer option for extra cooling in either city traffic with lots of stop and go or field work in which the truck would have to idle for long periods.
The five blade fans with irregular spacing came with big trucks with the 261 or 3/4 and 1 tons with the heavy duty cooling option (at least for the TF series). The five blade fan fits inside a shroud that fits into brackets on a radiator with a wider, higher capacity core. The irregular spacing looks goofy, but it has something to do with creating better flow by breaking up the wind flow.
When I converted to A/C, the four blade fan on my '57 235 needed help. I installed a 19" GM optional six blade fan. It helped quite a bit. Then I came across a '57 6400 that had come with a 261 that still had the irregular 20" five blade fan and the shroud. The radiator was shot, but I salvaged the brackets and mounted the shroud on my radiator and installed the five blade fan.
I don't notice any difference between the six blade/no shroud and the five blade/shroud performance.
If you can find a shroud, I'd use the five blade because they were designed to work in tandem on the 261. Without a shroud, I'd use a six blade because it was not designed to require a shroud.
You won't go wrong either way, IMHO.
Matt
