First series '55 is the same as a '54, so the short pump would be needed if everything else is stock.
The problem you are describing shouldn't be there. What are you using for a fan blade? are you using a spacer between the blade and pulley?
When installing the later engines into these trucks, the centerline of the pump is lower than original, so having the fan blade overlap the lower tank is typical. It shouldn't present a real cooling problem if everything is good shape.
Moving the radiator forward and trimming the shrouds is another option. Keep in mind that if you get too far away from the radiator, the fan isn't going to do much good without adding a fan shroud. Moving the radiator forward would allow you to use the stock late '55-62 water pump and pulley setup. Thats what I have in my '53.
As for adding an electric fan, I believe a 16" will just fit on the radiator, but how much clearance you will have between the pump and radiator I can't say for sure. It may be necessary to mount it a little high so the fan motor can clear the water pump. It is definitely preferable to use a puller rather than a pusher.
I believe they typically draw between 20-30 amps, so I doubt your generator will be able to keep up in a worst case situation. Switching to a one wire 55 amp or higher altenator would be advisable, in my opinion. Any vendor that sells electric fans should be able to give you proper specs and dimensions of their fans.
I have seen a plate adapter available to use the 53-54 style water pump on the newer engines, but I have no direct experience with this, but it would get your alignment issues back closer to the original first series '55 specs. Try Ebay for this plate.
