Hmmm, interesting....i just rebuilt the heater box from my AAR, and it also has the nylon fan. I robbed some parts from another e-body heater box for my resto, and while I was at it, I checked operation of both blower motors while the boxes were apart. The spare was equipped with a metal fan.
My results were quite the opposite though - the metal fan was tweaked and ran out of balance, where my original plastic ran nice and true.
I tried straightening the metal fan - it seemed very tender and very easy to bend, but since the plastic fan ran so smooth I ended up putting it back in.