ADMIN NOTE: The following was split from another (now locked) thread that started out trying to cover both 67 & 68 in the same thread
The following applies to 1968 Model Year:
GT engines: J-Code 302-4V, S-Code 390-4v, X-Code 390-2V, R-Code 428-4V Cobra Jet (All had 9 inch rear)
The J-Code was also offered in a non-GT model w/ 8 Inch rear, the S-Code 390-4V was also offered at the beginning of the model production without being at GT. The GT option became mandatory with the S-Code 390 post strike.
The disc brake option remained only for the J-Code (possibly X-Code), but was a mandatory option on the S-Code and R-Code cars.
The GT included GT Badging, C-Stripe, Fog Lights, Chrome Engine Components on 390 and 428, Styled Steel Wheels with GT Center Caps, F70-14 WSW Tires (Cobra Jets received RWL tires), dual exhaust with chrome extensions, pop-open GT gas cap, improved suspension.
May have missed a few things, I'll add as others provide input or I remember something else.
Danny