Cracks like this usually mean that the headers were made with stress build in. Just Mig weld them and they will be fine. Have the welder extend the weld 1/2 inch beyond the crack. If the welder uses the Pulse welding method with high current it will achieve good penetration and reduce heat.
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.