They apparently built them both ways, the short pump was starting to be phased out in around 1969 or so.
Typically you will find that there are many c10's and K code trucks that have the long. I have actually seen both set ups in stock form.
The ones with the long pump typically did not have AC. the same power steering pump is used in both pump configurations...
The passenger side alternator bracket is actually a superior system. My old 1972 Blazer had the drivers side and had AC and was considered the "Heavy Duty" version that Chevy called the "Heavy Half".
When I put the TPI Corvette in it (1992 vintage) I switched it over and used the AC compressor from the Vette, also needed to use a long pump and serpentine belts.
I bet if you were to ask for a pump at the parts store, they will ask long or short... Just a suspicion.
GT