Running without a stat will take a long time for the engine to warm up. Once it reaches operating temp it will continue to climb until it pukes. This will occur more quickly if the car is stationary. Just install a new stat and go from there. Since you mentioned that its the original engine and water pump is there a chance there is corrosion in the engine and pump that impedes its performance? Investigate.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.