One last thought.
Do you have an overflow tank?
It is the only way to insure that you have purged all the air from the system, and a recurring air pocket in the block could account for the initial spike.
I used an (repro) original style metal tank on the firewall, and properly modifies the cap.
One note, disregard the instructions to install it empty and empty it later. Always install it 1/3 to 1/2 full to allow the air to escape and coolant to be drawn back in.
No, Jim, I only have an overflow line dumping to "the world" at the bottom of the radiator. Interestingly, I started the truck tonight, and slowly creeped around the farm in first gear for about 30 minutes, and it never got to 220. 190 maybe, but at idle speeds up to maybe 2000 rpm, it never got HOT.... Hmmmm
