To be truthful, I don't know. However, two years ago I helped a friend install power disc brakes from a 67 Cougar into a 67 Mustang. That's when we found out about the 'similarities' and 'fits' between years. Being that the 67 and 68 drum set-ups are the same, I would assume that the disc brakes would be a bolt up. There are several areas that might prove to be difficult but not impossible to overcome: pedals - 67 disc and 68 disc are different but aftermarket pedals are mostly the same; proportioning valve - early 67 valves were located at the rear axle, late 67 and all 68 are located at the distribution block near the master cylinder, yours should be part of the kit and the lines are pre-bent are made accordingly; disc to spindle - should bolt up using the same holes as drum; power brake master cylinder - we needed to drill an additional hole because the power uses a stud at the bottom as an additional attaching point; distribution block - some use one bolt and a key slot, others use two bolts; vacuum from intake manifold - had to find the correct fitting only because we were going for a concourse restoration (not a problem).
Bottom line - go for it.