That "screw gun" noise- - - -does it sound like a small impact wrench, sort of a rapid hammering noise? If so, that's entrapped air in the fluid going through a pressure regulator valve. That indicates the fluid filter is sucking air along with the fluid from the pan, indicating an air leak on the suction side of the fluid (not fuel) pump. Once it's trapped in the pressurized fluid, the air has to go somewhere and the most likely place for it to exit is through the regulator valve.
If the shop is actually calling the item they want to change a "front pump", RUN, do not walk away and don't let them work on the transmission again. An aluminum PG has only one pump. The cast iron models had two pumps, with the rear one driven by the driveshaft so the vehicle could be push-started. Those pumps were eliminated on most vehicles by about 1958.
The TH 350 swap is a Band-Aid fix for someone who can't fix what you've got, and given their track record so far, are you willing to pay them to tinker some more?
I'll need to look in a few hidey-holes, but I believe I've got a very detailed maintenance manual on Powerglides stashed away somewhere. I'd be glad to copy and/or scan some pages if it will help you understand what you're dealing with. Good luck!
Jerry
