I have my distributor vent nipple connected to the air cleaner so the engine vacuum draws moisture out of the distributor. I have a 79 distributor on a 77 2F engine. I think the other nipple I have run through the firewall to a fuel filter in the passenger compartment.
That looks like an FJ60 fan to me. Not sure if it's bigger than an earlier 2F fan but wouldn't be surprised.
Looks like a later 2F fan blade since it has the side-fins on each blade. As Subzali said, likely larger in dia. I don't think OEM fans for a '78 are unavailable - you might try Rock Auto or the dealer. Since old fans can fail and fly apart, a new one might be wise.
Thanks for the explanation on the distributer vents. That makes sense on pulling air through the distributer to keep it dry.
I measured the fan shroud and fan blade and they are both close to 17.5" in diameter. It does look like the OEM/Aisin fan is still available and I think this is the correct size for my shroud as it would be just under 17". Thanks for the tip on that - I assumed they would be no longer available.