Vacuum Pump Alternatives
I'll start by stating I am NOT endorsing any of the approaches I am listing below. These are options I am exploring but have not yet tested. Proceed at your own risk
NAPA still lists two "rebuilt" pumps (call store for availability):
Delphi "DEM NLVP1697" - 1 year unlimited mileage warranty
NAPA Branded A1 Cardone "NVP 641201" - 1 year 18K mile warranty
Usually the drive mechanism on these pumps (the tower/column) rarely fails. Changing out the pod as has been suggested seems to work for some. The "NLVP5041" seems to have a pod very similar with the inlet tube angled slightly off as it is intended to run on a Ford belt drive unit.
The pumps for similar year Chevy vans seem to have a similar pod again with the inlet angled farther off than the Ford unit, but I believe it would not provide any clearance issues. YMMV
As FYI - Delphi part no. for the pod only is/was POD1697. Does not appear to be stocked anywhere.
Now - here is what I am most likely to go with when my "distributor drive" pump(s) fail next time. Again, no warranty is implied since I have not tested this.
A newer electric vacuum pump technology has been on the market for some time. Instead of a noisy piston pump design these pumps utilize a rotary vane design which is much quieter. All vacuum assisted power brakes require a minimum of 18 inches mercury vacuum so all of these pumps "should" provide the approximately 20 inches we need for properly shifting our old girls.
There are several GM/Delco parts that "should" work. Examples are:
ACDelco 20939309
ACDelco 20804130
Looks like GM is buying from Hella for these.
I would put together a kit like this one used for gasoline powered vehicles with aggressive cams:
"
Fixing a Power Brake Problem Using An Electric Vacuum Pump"
"
Master Power AC9001K"
Lastly, since the "distributor drive" is required to run the oil pump I would either replace it with a speed sensor only unit used on engines with a belt driven front mounted pump, or machine a block off plate to bolt up in place of the pod if the pod became noisy. I would leave it in place if not.