But why not replacing the 727's with 757's?
PA operated a HUGE fleet of B727... they didn't need the capacity of the B757, nor did they really have the funds to purchase it.
The A320 doesnt have that much cargo space, they could had stayed with the 737-400.
PA had pretty much phased out the B732... the operated a very small number, and mostly within Europe. And while I'm no expert, the A320 can carry certain cargo containers that the B737 cannot.