Sounds like an easy fix now you know the secret!
Let us know how it goes, got to love a picture.

 

Okay lets back up and get on the same page(at least me )
yes i was reffering to the earlier one piece t stat. Now that here is you most likely issue, 2 actually

1) you installed the stat correctly with the needle into the small stainless cup in the t stat cover, but, the notches that you rotate the stat into holding it in place are plastic and they break releasing the t stat and it creates a issue like number 2 below. This is the primary reason i do not do t stats alone anymore but replace the whole housing.
2) the t stat housing is a internall crossover pipe. The large metal disc of the t stat sits against a opening inside that closes off the crossover pipe between the heads. These tend to crack and a piece will break off creating misflow inside the engine that makes it run hot like you describe when driving down the road. This is because the coolant doesnt flow properly between the heads and flow is greatly reduced in the engine as well. We see this when a tech replaces only the t stat as you did and a few days later the customer returns complaining of overheating. We remove and viola' theres a piece missing inside. This is another reason to replace the whole housing
there now im up to speed

 

Brutal - The thermostat housing is cast aluminium as are the tabs that hold the thermostat in place. The 'bent bar' piece that holds the thermostat in place on the cast housing is also metal, in fact in this type of thermostat there is no plastic whatsoever? At least this is the case on my XJR with the same 4.2 S/C engine.

The thing coming apart would only be an issue if the thermostat were not locked into position properly?

I will be posting a step by step on replacing the thermostat soon for anyone else doing the swap.

 

You know im just gonna go back home to bed. Ive gone from thinking early style 1 piece t stat to the later spring loaded in a plastic housing only on the na engine not the sc. Im clearly in need of a vacation thats still a month away

 

Alright, I got back from my holiday trip and pulled the thermostat out and pointed the needle in the correct position (away from the engine). drove it around for an hour at various speeds 50-80 mph. She never got above the halfway mark. Thanks for the picture!!!! The only thing I would add is to put a shield over the belts, so you don't get any antifreeze on them. I used thin plastic painters tarp. It comes in a roll and I just cut enough off to tuck under the thermostat housing and over the belts. Don't forget to remove the plastic shield under the engine bay. Then when everything was put back together, I wet down the front of the engine (before you start the engine) to make sure you get all the antifreeze off the engine.