I can't say that I "Know" the shorties won't fit, because it's not something I've ever tried, but it's my "opinion" they WON'T fit on the '68 full size chassis. I have a '66 Galaxie 7 Liter ( 428 car ) and, with the help of my good friend BradFord, we put the '65-'67 long cast headers on the car. Here's the reason for my opinion - the forward two runners on each side, cylinders number 1 & 2 and numbers 5 & 6, come very close to the suspension upper A-Arms and A-Arm shafts. As we know, Ford made a total change to the chassis of the full size cars starting in 1965 and that required a change to the long cast header design from the earlier '63-64 long cast headers. The main thing that is different is Ford made the forward cast manifold runners come back in a straighter and slightly higher shape than the earlier design. Take a look here at the picture I grabbed from the current Ebay auction and look at how straight the forward runners come back:
Now look at how much more gentle the runner curve is back from the forward two cylinders on the cast shorty headers ( Photo from MustangTek ):
Now I will say - the pictures here are not to scale with each other and are from different angles, so you can't just go from pictures of course, but like I said I do have experience with the long cast headers.
One other thing to consider that I have no direct comparison experience with, but Ford changed the power steering system totally as well starting in 1965 and the power steering gear box may also interfere with the earlier shorty cast headers.
It would be nice if someone had a bare chassis under restoration available where they could do a trial fit.
