My first thought was to check your MC push rod length. This is a standard problem with new MC installs in our Panteras. Just a little too tight, heat things up, and now you have partial push rod movement.
As for a heat shield, aluminum is a GREAT heat conductor. It will soon radiate off the back side whatever temp is hitting it on the front side.
Stainless is not a good heat conductor and would serve you much better.
Get a flat sheet of stainless sheet. Make it as wide as the space your 3 MC take and deep enough to extend out far enough from your MC fire wall section to cover the header pipes. Add a 1/2" 90 degree bend on the fire wall edge and attach it to the fire wall.
Larry
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.