Gudmund, many 351-C stud girdles will fit under cast aluminum 'DeTomaso' valve covers. The only girdles likely to cause trouble are the more bulky 'Grand National' racing types once made for NASCAR. You will need to remove the stamped steel baffles from inside. I have run a different make of stud girdle (no longer available) together with full roller rockers under those covers since the late '80s with no problems at all. I also use early 'large' PCV grommets for both oil filling and PCV and have no oil leaks around them. If there's any sort of clearance trouble with any cast valve cover, I suggest trimming the girdle to fit, not the valve covers, or gluing two valve cover gaskets on top of each other to cheaply raise the things up 1/4" for clearance.
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.