Stop driving it, it has been overheated enough that you are going to toast that engine.
No more parts or anything till you determine and repair the problem.
If you want some help today, send me your number and I'll call you back walking you thru what to do next.

 

You are possibly going to need a radiator as well. Previous owners like to dump in stopz leakz to "fix" overheating problems. It gunks up lower rows in radiator, as does old Dexcool coolant and use of tap water. If radiator fins at top are 10F hotter than ones on the bottom, then you could have a partially blocked radiator. Can't flush the D2 ones very much, to thin inside.

Don't know how else to put this, but if you depend on just the dash heat gauge you can toast the engine without knowing it.

 

I'm surprised the head gaskets haven't burst open

 

For those unfamiliar with the gauges, note needle position in these pix. For better diagnostics use a scanner, no need to go above 215F (say to 235F) if you are troubleshooting. Some go by when the electric fan kicks in, at 212F; but some find out too late that their electric fan is not working....

And while it can go to 215+ for short periods, running at that level for extended time is not good. Normal highway cruise level ground no trailer should be in the range of 180 - 200F, depending on thermostat. The gauge allows people to drive around at really hot temps, thinking it is normal.

 

Your radiator is plugged.
Remove it, flush with water, fill with CLR, let it sit overnight, flush with the garden hose and try again.