You have a ways to go before taking anything apart. The plugs look quite rich, they should be tan in color, put new ones in, make sure to use anti sieze on the threads. A reading of 105 does not in itself make it a headgasket problem. Given the history of the engine I would more likely suspect a "stuck" compression ring. I've run engines with less than 90 lbs. and they run o.k. so your readings except for the one cylinder at 105 is healthy so you need to concentrate on that cylinder. Get some Marvel mystery oil, use a clear tube hose (available at ACE Hardware) and put in enough to cover the top of the piston. Check it after 24 hrs. to make sure the level is going down. Keep doing this until you've gone through the bottle. Don't worry when it drains into the oil it's still o.k. to run it and will actually help free up the other piston rings. Run the engine for 15 minutes with the water pressure tester on the overflow cap. While running check for a blip from when that suspect cylinder would fire. If the engine will run without overheating I would run it around for awhile. Yes, the combustion gas test is the ultimate test for a blown head gasket.