I've made pvc blades out of the 6" pipe and it seems to me that 6" wide at the root is too much curve (excessive drag) hanging over.  I think that would be like 3" wide.  The blades I made were 3.25" at the widest point.  

The cogging of those motors can be a real pain I know. Small mills have to be able to start on a feather and this is difficult to do with 4 and 5 footer DC motor mills.  
I would try 5 thin  pvc blades (same 54" diam) myself.  It would give it  a lot more start-up power. (but it won't mean any more power though than 3 blades)

The other thing that can cause big start-up problems is balancing. I find It is impossible to balance the blades with the DC motor itself due to the cogging. I always just use a bronze bushing in wood separately, to balance the prop more accurately, or you can take the rotor out of the treadmill motor and hold on to the two ball bearings and balance it that way very well also.