I can't believe there are still people rebuilding these things...unless it is going on a show queen.
Because many of us, who have kept them properly maintained over the years still love how well they stop!
There's only one place that has new GM powermaster motors & that's cars.com
They sell for $325 or you can buy a Rebuilt complete unit from several of the vendors on here. There is no such thing & never was a rebuild kit for these motors.
GM part number 18020067 .. new number 18038510 was the rebuild kit and some time ago Hank Terry offered rebuilt pump E-H units. Not sure if he does any more.