Im in the same state as you are, except my car has nearly no mods done to it...
You just spent all this dough on your car and now you want to sell it? I can understand its given you problems but I would hold off on making any sudden decisions.
Even if you buy a "newer" car such as a 99 Cobra you will still run into things breaking and stuff to pay out of pocket... It isn't a new car after all...
So it really comes down to nothing lol... You want a newer car, go buy it.
I'd personally keep the GT you already have since you've done all of this work to it for a while longer, pay it off and then upgrade...