Krector 88,
These trucks are getting old and likely your head gasket was already on its way out. Cranking on the head likely was all it took to squeeze it or crack it to a point of failure. You did nothing wrong, just had to deal with a problem that appears to have created another problem.
You possibly could have cracked the head but I have never heard of that happening from an injector or glow plug on this or other forums. But if the head had a hairline crack cranking on it could have made it worse.
If you are confident you did the oil cooler correctly and you feel the bottom end is still in good shape. Then you should pull both heads, have a machine shop check for cracks and bad valves, replace the valve seals at a minimum and reinstall the head with new head bolts. Also chase out the head bolt holes. This forum is 50/50 on if easier to pull the engine or do them in the vehicle. I think a lot has to do with your experience, working conditions, and tools available. IE do you have a heavy duty cherry picker and engine stand to pull it and work on it. Do you have a clean space inside to work or just a dusty old barn with a dirt floor. Or do you have a buddy who can help pull and replace the hood and heads while the engine is in the truck to make the job easier.
Just some other things to consider. If you did the water pump recently a couple bolts go through into the engine chamber and require silicon or other sealant or can leak coolant into the block. If coolant, rain, water is leaking onto the top of the engine it can get in from places like your cdr valve grommet. These trucks suffer from cavitation if wrong coolant is used and can get a hole from the cooling jacket into a cylinder and leak coolant into the motor. These cases are rare but something to look at when the heads are off. Based on your story I doubt this applies to you.
Also keep in mind doing the heads is some work and money but you will have cleaned up a bunch of other issues (Exhaust gaskets, valve cover gaskets, intake gaskets, glow plug inspection, etc.) and made the truck more reliable. You likely will have higher compression and more power if any valve work is done.
We all have to keep in mind diesels are not cheap to buy or repair but once repaired they are stronger and more durable than gassers.
Sorry to hear about your woes and let us know the outcome.