[NAJ]

#2 spark plug is much cleaner than the others which indicates that may be the problem cylinder.

The "Block Tester" will only tell you if there are exhaust gases in the cooling system.
Compression will not be affected by a headgasket water jacket sealing issue.

The only true way to tell if the headgasket has a cooling system/cylinder leakage is by performing a "Cylinder Leak Down" test.
A Leakdown test will show you if there are any leaking headgasket issues and which cylinder(s) are affected.
Do you have access to an air compressor with a minimum of 100 PSI?

How to do a leakdown test | Mobil™ Motor Oils

Before you tear down the engine, pinpoint where problems are by using these three steps to do an engine leakdown test.

www.mobil.com

When you rent the block tester (you have to purchase the fluid) you may also want to rent a cooling system pressure tester.
Use the pressure tester to test for external leaks, leakdown of pressure, testing the rad cap and...
Connect the tester to the radiator, do not apply pressure, start the engine, pressure should build gradually, if pressure builds quickly there are exhaust gases in the cooling system.

https://www.autozone.com/loan-a-tools/loaner-blown-head-gasket-tester

https://www.autozone.com/loan-a-tools/loaner-radiator-pressure-tester/p/powerbuilt-cooling-system-pressure-testing-kit-22-piece/449951_0_0

 

[NAJ]

 

[NAJ]

All I can say is, WOW!
Glad you found the issue.
How are you going to clean the water jacket in the block?

I looked in the Chrysler Parts Catalog for your manifold question, they show the manifold with the area you are referring to, however, they do not show anything being attached there, each catalog from 86-92 shows the same area on the manifold.
It appears to be a machined surface and looks clean as if something where there.