I'm assuming this is the car??? If it is, I've never seen a '68 GTS with the "GT" emblems on the right corner of the trunk lid and the "DART" emblems on the quarter panels ahead of the side marker lights unless it was just a "GT".....but you say; I am working on my original big block 68 dart with a 440/auto that still uses the original cast iron big block exhaust manifolds..
.....I've never heard of an original big block GT Dart.......

What am I missing??? Is the car in this pic a real "M" code?? From the looks of it, it appears to be a '68 GT with a GTS hood......

If you're building a clone anyway and to avoid all this "is it legal" BS, just build it as a low deck and put 383 badges on it......