You can use either pump on the engine, you just have to match the pump to the accessory drive belt style as 50vert noted. 

HudginJ3 wrote:

I thought if you got a reverse rotating water pump you had to change the timing chain cover to match the water pump.???

No, as long as the pump will bolt on to the cover regardless of rotation you're good to go. A word of caution, as was mentioned as far as the pulleys and brackets must all match. In 1970 Ford changed to the lower hose on the driver's side, a 4 bolt pulley on the harmonic balancer and the timing marks in a different location. This must all match. Timing covers up to 67 had a cast on timing mark and don't lend themselves to the 70 and later pulley arrangement. The front timing chain covers were made to be universal with bolt on timing pointers and these can be retrofitted with the appropriate bolt on timing pointer
 

If you are replacing an existing pump that was hooked up and working, all you need to know is if you have a serpentine belt where the SMOOTH side of the belt turns the pump, then you need a reverse rotation pump.  If you have a v-belt or even a serpentine type belt, but the grooved side of the belt turns the pump, then you need standard rotation.

 

It seems the covers are slightly different, but I've heard that so long as you use the correct water pump backing plate it doesn't matter.  Of course I've also heard that a mismatch can cause overheating.  I'd ere on the side of caution and match the cover to the pump.  A new cover is available for either application (some I think may also be universal), and aren't terribly expensive. 

If you look at the covers side by side there are two apparent differences as it pertains to the water pump.  The first is that there is an additional bolt hole on the early style timing covers.  I can't say the result from simply not using the hole if installing the later style pump without it.  I would think it wouldn't be an issue because Ford clearly thought it wasn't necessary when they deleted it on the later covers and pumps.  Otherwise the bolt pattern is the same. 

The second difference is the ramps going into the water ports are in different orientations between the two covers.  BUT, this is where I think the back cover of the water pump makes the difference because the holes in those covers are simple circles.  The water flow path through the engine is the same for all Ford small blocks.  So long as the circles on the pump cover and the ports in the timing cover line up and nothing is occluded I don't see why it wouldn't work.  However, I can't say if that is the case or not for certain.  I would say that if you were contemplating using a RR pump on an early cover I would advise taking the back plate off the pump and lining it up with the bolt holes on the timing cover and checking the water passages.  If there's no restriction created by the match I would expect it to cool just fine.  If there is a restriction I'd swap the timing cover to the later style to avoid a possible reduction in water flow and potential accompanying cooling issues.