From the photo you posted it seems like maybe the headers are just mounted on the opposite side from where they should be. I would first remove the existing system and move the headers to the opposite side to see if they fit. If they do then you only need to deal with mufflers and tail pipes.
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.