What I normally do is grease-up the driveshaft flat and stick the key to it. Make sure the key is aligned straight and in the middle of the flat. Use a Sharpie and scribe a line directly above the key.
Make sure the impeller is lubed-up, too (dish soap works, I use marine grease). Line up the impeller keyway with the key. Insert the impeller into the pump's stainless cup with a clockwise twist, while turning the driveshaft to keep it lined up with the impeller keyway.
If you're successful, the impeller will drop right in! Otherwise, it'll hit the key in the wrong spot and push the key out of alignment. You have to make sure that the impeller is engaging the key, by turning the driveshaft clockwise; the impeller should follow and if you wiggle the driveshaft ccw/cw you should feel a little slop between impeller and key.
Some folks use a couple of tie-wraps around the impeller so it'll drop right in without the impeller vanes touching the pump insert. Or a small chunk of pipe, slightly smaller in O.D. than the pump insert. It'd have to be thin-wall to fit, though.
I've done the insert/twist method enough that it works pretty well for me; but there's always a stubborn one! Be sure to check the fit of the impeller, without the key, to ensure the impeller is the correct I.D. for the shaft. Especially since you've upgraded to a newer driveshaft. Pretty aggravating it don't fit, after all the trouble of trying to install it!
HTH..........ed
