Both standard hoods need modified for the R/A set up. This includes R/A scoops.
The '68 GTO had Ram Air II not R/A III. If your '68 has the R/A III dimension ontop the carb 5" the R/A III bottom pan should fit. I think it depends on the carb's top dimension. I don't know if the 68' Q jet has the same top dimension as the 69-70. Another thing to consider is the intake, if it's stock the height should be ok, if its not and sits too high you will have a fitment issue.
Measure your top of the carb, if its 5 or 5-1/8 or so " the R/A III should work.
There is a brace on the under hood that needs a part cut out for the upper pan to fit. Some have done this freehand but there is a template for this.
It should work. The Beast is a 69 that wasn't originally a Ram Air car, but I did install an aftermarket Ram Air III system on it. One thing that may be a (very small) issue on the 68 hood is the screw holes that anchor the upper ram air pan to the hood. All 69 hoods had some of these these holes drilled and in the correct locations, regardless of whether or not the rest of the system was on the car. There were only a few that I needed to drill to complete the installation. I don't know if the 68 hoods will have any of them or not. Having them there helps to correctly locate the upper pan. As far as cutting out sections of the hood bracing to allow installation of the upper pan, I did buy a set of the templates to mark the needed areas to cut out but having done it now - you really don't need to buy the templates. It would have worked just as well if I'd positioned the upper pan over the screw holes and traced around the front of the pan where the two inlets are. Those are the only areas that need cutting.
Here are some photos from when I did mine:
Stock 69 hood - the small hole in the bracing that's centered between the two "scoops" is one of the pre-drilled mounting holes I was talking about.
After cutting and drilling additional holes. The holes that have the dimple/countersink around them were already there. The others I had to drill myself. In this photo all the cutting is done. The uncut section that's marked on the right hand side of the photo, despite being marked by the template, didn't need cutting after all. This is why it's good to go slow and test fit things as you go along. Vendors sometimes make mistakes.
Test fit of upper pan
If you're trying to retrofit a 69 kit onto a 68 and aren't sure how to locate the upper pan, I'd do it this way:
Install the intake, carb, lower pan, foam seal, uncut hood - and nothing else.
Put some double-stick tape on top of the foam seal, or double some tape over so it's sticky on both sides.
Close the hood - the tape should stick the foam seal to the underside of the hood.
Raise the hood, then mark around the outside of the foam seal onto the underside of the hood.
Now you know where the upper pan needs to go in order to mate up with the lower pan and seal.
I did mine with an air powered cutoff wheel. Where the braces were right up against the hood skin I went really slowly and didn't cut all the way through, but just enough to score the metal so that I could bend it and break it.