If your car is close to stock except for the changes you mentioned (depending how radical the cooling system upgrade was) I'd say changing the bumpers would detract from the value.
BTW don't worry about the questions. That's what we're here for.
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.