Stenmark,
You might try experimenting with various engine simulation/modeling programs to see what different header designs do to your predicted power curves. There are also options for others to do the modeling on your behalf. For example Burns Stainless offers a service using their X-design software based on the details of your engine. I believe they charge $75. May be worth the expense if building your own custom exhaust.
Dave
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.