I don't think anyone can offer much in the way of help based on a "pretty" picure.
Since you didn't attach the actual *.sldprt file the only suggestion I can make is -
go through the built in tutorials
search Google on Ed Eaton Curvy Stuff Tutorials DiMonte Group
and Mike Wilson
and purchase a book on SolidWorks surface modeling.
I cannot say for sure that this will work, but I would start by trimming back some distance on each of the surfaces, or just one, and creating a new lofted surface between the two so that they are not disjointed as they are currently.
Depending on the complexity of the part and geometries that you are working with this may or may not work.
Steve, your problem is that you've tried to implement a very low level parameter like overall length at the highest level of your design.
You can try taking a chunk out of the rear hood and then doing a combination of fills and boundary surfaces to merge the two together, but you're not going to get the curvature to match and the result is going to look like you cut a hood in half and tried to bondo the two halves together.
Add to that the fact that SolidWorks doesn't include a lot of the tools needed to construct and modify surfaces. You probably need something like Rhino or the PowerSurf plug-in for SolidWorks to be able to resize the hood.
What you'll probably have to do is redesign the hood from the cut to the windshield. Draw in new rails from the front half of the hood and use back end to give you landing points for your guide rails and reconstruct the surface one patch at a time.
Now, a few months ago, I was in a similar situation where I was modifying someone's Creo surfacing model of a pair of Sunglasses and I was doing the work for would casually throw in there, "You know, I'm looking at these Oakly Frogskins and the arms are a lot shorter," "I'm looking at these frog skins and the ear piece comes down lower," "I'm looking at these Frogskins and the lenses are a lot flatter" and I'm like, "Dude, I'm working with the Pro E model you supplied me. If you want to change that stuff go back to your Pro E guy." I did actually try making the mods myself.
By trial and error, I was able to make the frames thinner using the Scale feature. I was able to change the overall length of the arms using the Deform tool and I was able to change the curvature of the earpieces using the flex tool. In each case, I had to tweak my settings,check the resulting surface for ripples, singularities and continuity errors and iterate two or three times.
You have destroyed the integrety of that model by just cutting 8" out of it. I'm sure it was easy and fast but the guy that created that model and the next guy that has to work on it are not going to be too happy. Faster is not always better. Take a little time and see how it was created and shorten up all geometry that applys. Its probably just changing the length of a drive curve or maybe the distance of a sketch plane by 8". Don't be a hacker! LOL
"Nobody ever remembers how long it took."