i just pulled the drive apart, waiting for tools to remove the impeller and take it to the next level.  First guess after looking at the impeller, it is a B and appears to be pretty much sand blasted, also the clearance has to be severely worn, I can move the shaft up and down, side to side, pretty sloppy, have not actually measured for clearance.  I plan on doing complete rebuild on the drive, just torn on weather to order A  or  AA impeller.

When serving USCG boating safety team in '68, '69, we would work the River between Laughlin and Havasu and there I fell in love with the Jet Boats...  Now 50 years later I finally own one...  While escaping the Montana winter at Pirate Cove I bought a 1968 Hallett 18' that others made few changes over the years, with exception of the pump...  Now has a 454 BBC and the engine runs sweet, the pump is not only worn out but still has a "B" impeller...  We look forward to cruising the river next several years and I just removed the pump and am about to order rebuild kit and am asking for experienced Jet Boat owners to suggest correct impeller...  My goal is to cruise 35 to 40 MPH at 2800 - 3000 RPM ...  My guess is the engine is at 300 HP and we will be two person boat and never plan on using to pull anyone out of the water, unless by throwing them a flotation device :-)   Looking at charts, looks like I should get an "A"  or  "AA" impeller...  Any input appreciated,  Thanks in advance,  bagsworth