OEMs are 50 watt low beam/60 watt GE #6014 sealed beams.
Some folks opt for the Sylvania SilverStar bulbs part number is "9003 ST".
Brightest option appears to be these:
http://pantera.infopop.cc/eve/forums/a/tpc/f/3350045562/m/1930077873
Does your car have the headlight relays?
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.