It might be a good idea to pick up a set of oil galley cleaning brushes. Both Amazon and Autozone have them listed for around $50 for the complete set. That will allow you to make sure every oil passage is clean and clear of the paper towel fibers. I'd also check for wristpin knock or lifter issues while you have everything down.
Fingers crossed that you find nothing serious causing the knock.
John

Hollow knocking sounds from inside the engine are almost never a good thing!

It used to be when a turn signal would blink, but the dash indicator was out indicated one bulb (front or rear) was not working, and if the dash indicator was on, but did not blink, it was the other (front or rear) bulb that was not working properly. I would check the bulbs in the rear to make sure that they are dual filament and that the sockets are not distorted. I have had a bent socket allow one of the bulb contacts to bridge the socket contacts. I have also seen a socket that was worn enough that the bulb could be put in 180 degrees out and that would bridge the contacts as well.

Worse case I ever had were totally worn shoes rusted to the drum. I actually chiseled off the back of the shoe retainer pins and was able to pull the bottom of the drum off with the shoes still inside, then lift and twist enough to get the springs loose enough to pull the entire assembly off.

You can check Harbor Freight for brake tools. When I worked at our local store a couple of years ago, we had individual tools and a set for drum brakes that included the spring pliers, shoe retainer spring tool, and a couple of adjusting spoons. Not Snap-on quality, but should more than suffice for DIY'r.

I've been a fan of Copper Coat Gasket Compound for years. But I only use it on the rocker cover side of the gasket.

You could try running both GPS unit wires to the battery to see if that solves the problem by isolating the GPS from the vehicle electrical system. If so, then I would consider leaving it wired direct and adding a toggle switch for the GPS power circuit. If direct connection does not fix the issue, the problem may be in the GPS unit itself.

I wonder if some brake fluid would soften the brittle gasket making it easier to cut?

See if you can find a longer lug bolt, or piece of "all thread" that fits your drum. Cut a length of the "all thread", or cut the head off of the lug bolt and screw the threaded portion in a few threads to act as a support while you screw in the other lug bolts. Remove the piece and replace with your last lug bolt, then tighten all.

Stewart, I have rebuilt many Quadrajets in my earlier years as a mechanic. I do not recommend using any blast media on any part of a carburetor. The metal is soft and it is likely that any media, be it glass bead or walnut shells, will deform mating surfaces which will impact operation. I have always used a good "dip" product, such as ZEP (not sure if it is even still available), followed by a wash with hot water and then blow dry with compressed air. The loss of anodizing should not have much, if any, effect on carb operation, unless there is pitting on any of the internal air flow surfaces. I don't think applying any paint type finish to the outside would be feasible since it may allow heat to build up internally which could affect operation.
Just my two cents worth.....
John