It sounds like you are headed in the right direction. After you run the tests you will have a much better indication of the condition of the engine. If it is a blown gasket it will likely just be on one of the heads (but it could be both). If the gasket is blown, you will have to drain the coolant, remove the intake manifold & head to determine the mode of failure. Draining the coolant from the block can be a real challenge on an older engine if the block drains have never been removed. I had to drill mine out and install drain valves. There will be some old gasket & sealant to be removed. As has been said, you can use a manual scraper. If you don't like all that scraping and have a compressor, you can use an air scraper. Then you will need to inspect the block & the head for problems. Check to see if they are "true" by using a straightedge on the machined surfaces. If they are true and crack free you can put everything back together using new gaskets and possibly some light sealant if you like. Try to keep the old scraped off crud from getting in the engine. The correction of a blown gasket will likely help the overheating problem significantly. There are chemical tests available which detect combustion gas in the cooling system if you need it, but what you are planning probably will show a blown gasket also.

 

I figure if one head gasket is blown, the other can't be far behind, so it would be a good idea to do both at the same time. I believe the hydrocarbon test done by the radiator shop is the chemical test of which you speak.
Hopefully the rings, pistons, and block aren't shot, I'm definitely not set up to be yanking an engine.

 

Update: I checked the compression on all cylinders. They're all between 120 and 135 PSI, which is a range of 12.5%. All the spark plugs tips look fine, but the plugs have black grainy particles on the threads. Engine is firing on all cylinders. Seems like relatively healthy results to me, but what do I know? Is there any significance to these results, IE indications of the health of my rings/valves/gasket?

 

Comp is between 125-135 on all 8 seems pretty good I would be thinking possible water pump impeller slipping on the shaft, these are relatively easy to remove and ck, does she smoke,? And what color? If she does, if w/pump ckd out have u had the radiator ckd for full flow? At e end of the day u may be Pulln ur heads, plenty of help here if u need it good luck

 

You should pressure test the cooling system and see what that shows. Compression could leak into the cooling system. If it is it should warm up when cold exceptionally fast. If it seems like normal it probably is.

 

Your compression values seem reasonable. Were there any of the lower ones (like the 120) adjacent to each other? I'd hold off on the head gasket job pending more testing. I'd normally think that the radiator & thermostat are O.K. if they are less than 5 years old. You could take the thermostat & put it in a pan of water on your kitchen stove with a known good thermometer. Heat it up & watch the thermostat open while watching the temperature. It should open fully near its rated temperature.
Is the overheating mostly at slow speed, fast speeds, or both?
Engine overheating can be caused by many things, some seem obscure, like retarded ignition timing, congested exhaust, and sometime it's a combination of things. Check as much as you can and report back.
The spark plugs seem reasonable except for the black particles. It sounds like carbon. Clean the threads and seats on the plugs and in the heads. Check the gaps and reinstall using some lube or anti-seize on the threads and seats.

 

the 120's were on opposite sides of the block. It's mostly overheating at low speed, though it has overheated at high speeds with a ~1600lb camper in the bed. I'm figuring on getting a coolant system flush and replacing the radiator cap (which was not replaced with the radiator) next.

 

Update: got new radiator cap. Took truck to the jiffy lube to flush coolant, they said my coolant is fine (pH test) and it would be a waste of money to flush. They also said it seems more like a faulty water pump than a HG. Bought a water pump, haven't installed it yet. drove about 20 miles total, temp outside is 67f, no overheating whatsoever.

 

Thanks for the update. If the coolant is good you can save it & reuse. If the water pump is at fault it would have to be something like broken vanes, loose impeller, or congestion. The pumps I've changed in my lifetime were always because of a seal leak and/or bearing failure which is easy to diagnose. Be sure to clean all the gasket surfaces well and use some sealant especially on the retaining bolts' threads that go into a coolant passage in the engine. The reward for not doing so will be getting twice as much experience doing the job again after a leak is discovered. You are lucky to have only 67F to deal with. I'm assuming that your Portland is on the west coast. Many of us have to deal with 30-40 degrees more in the summer. That is a real test of a cooling system.

 

You're right, I am lucky that I live on the Best Coast.
Jokes aside, it got into the high 90s/low 100s for damn near a month last year, and it's expected to do so again. That's why I'm getting things sorted now.