Brooklyn CT, Amherst NH, Thompson Speedway CT and Stafford Motor Speedway CT are a few of hunting grounds I've been to many years (decades) ago while looking for parts.
Or, before buying another gauge, if your existing gauge has the nylon/vinyl tube try disconnecting it and see if oil flows into a container while running.
Get some of the dye and a black light, or an inexpensive radiator pressure test kit. There are freeze plugs in front of the flywheel, after further review check the intake gasket at the back of the heads.
If you are working on the '78 K10, and if we are talking about the same thing, then it's not really a tapered washer but a tapered cone maybe 1/2" or more tall. The one on my '84 blazer fought me every .000005" of an inch on the way out. Try a lot of cussin" and don't throw any tools may help.