When you put thr prop on look at the indentations. If it is not rotated correctly it not seat all the way down. believe they both take the same bushings, so you can move it over if you want!
That was it!!! I've already got silt flying around my tank waiting for my filter to grab it.

I tried rotating it a couple of times previously with no luck, and the shaft isn't keyed until the last couple of millimeters, which makes it a bit deceiving. I tried a bit more carefully just now, and that was the answer.
Thanks Blurry.
Next I'll try the other new bushing in the 750 and see if that fixes the backward spinning...