I have a 1999 Mustang Coupe, I have dual exhaust installed on it and was wondering if anyone knew if the GT rear bumper/valance could be installed in lu of the stock bumper

 

yes

 

Yes it can, however you might need to bend the pipes down slightly or recenter them if the holes do no align properly with your pipes... but these are all minor adjustments that can be done in any shop, and aren't very hard to do yourself. Before you buy, you might want to find someone with a GT and check the clearance of your pipes and measure the distances between the cutouts. But if you're good with thepipe clearance it should fit no problem.