Why not swap the sending units from the old engine to the new one?
I had a problem with my 85 300D, that the Temp gauge would suddenly spike, then when I turned off the engine and turned the key back to the start position, it would go back to normal.
It was also jumpy. But when it spiked to max temp, talk about getting nervous.
My problem finally turned out to be the ground strap from the engine to the body.
Removed the strap and cleaned the contact points, and all was well after that.
Charlie
sent from my pos computer