I'm thinking the truck may have been neglected for the last 5 or so years as the oil changes were less frequent. Other than that I have receipts for oil changes back until 1995.

Here's some more cringe pics for you guys:

Wow, that sure is nasty!

OMFG that's vile 5h17 in there!

Ya might do a compression test to see if that might be what it looks like.

I was actually thinking earlier today it would have been cool to do a compression repair before and after starting all of this work. I'll have to settle for after.

So I drilled out and tapped the broken intake manifold bolt and the intake is now torqued down properly.

About 6 valve cover gasket bolts were finger tight and the gasket looked a bit leaky so when replacing it I confirmed that it was blown out in several spots. The valve cover gaskets are now both replaced and torqued in.

I got a new thermostat with the $30 plenum kit (confirmed piece of junk). I brought a pot of water to a boil with both of the thermostats in it and the one that was already in there opened up and closed just fine. The "new one" opened and the shaft came ajar so it's busted, I'm super glad I read a few reviews that it was a piece of junk and decided to test it. The one that's in there now is a stainless one that was replaced about 8 years ago and it still operated just fine so I'm going to leave it in.

I've started doing my final routing of the spark plug wires, and the electrical and vacuum connections are getting put back into place. I should have the truck running tomorrow and I'm excited to see the changes.

I was planning on changing the oil after getting it up to temp tomorrow, and I was looking at either adding some transmission oil or Magic Mystery Oil but I'm still on the fence as I've read it can do more harm than good.

The other option is to change the oil after idling as stated above, and then changing it again shortly after running it for a while. I have two filters and enough oil for two changes.

Thoughts?

I can imagine what the bottom of the pan looks like. It would be wise to drop the pan and clean out as much as you can with kerosene.

It's not "more harm than good", really. It's removing the sludge that is currently burying the leaks in the seals that have long been eroded from the crap floating in the oil. Seals within which rides your very expensive crankshaft that you don't want to abuse with the 5h17 embedded in those nasty old seals anyway -- so if you clean it and it leaks, you either find out how frotzed up the thing already was, or get an opportunity to save it before it gets outrageously expensive.

Sounds a lot like more good than harm, to me.

You're right that is logical and by not running anything through for fear of it breaking loose and causing an issue is just delaying the inevitable.

Would you drain the oil immediately after I finish the repair or should I run it for a bit first? I'll pick up some of that magic mystery oil.

There are those that would say some SeaFoam in the crankcase to help clean the sludge and baked on crap.
But there are likely just as many that would not recommend this.

[Edit] and there are those that suggest using a higher detergent diesel engine oil to clean crud.

And then there's me. I'd clean it out with diesel fuel. That's fun because it scares hell out of people.

Back to the last question: I'd drain the cold oil just in case crud has been dislodged during the work, then do whatever cleaning method, if any, is chosen.

That is the key concern... the dislodged crud is what has the potential to get circulated and block any small passage/orifice and stave whatever is down stream of oil... more so than the fines being an abrasive in dirty oil.
So whatever you choose, flush, drain, and repeat... anything that you use is going to be cheaper than doing a rebuild.