I am using the Revell 1/144 complex, which I purchased partly assembled, as a starting point. I didn't intend to scratchbuild any of it, but when I scaled it out I decided to make it a bit more accurate. I figured since I was gonna do that I might as well go ahead and add rails and the gantry stairs, which I feel are a prominent feature missing from the Revell kit. I intend to use the Academy 1/144 Shuttle Stack with a scratchbuilt MLP, I doubt (hopefully) anyone will notice the difference in scale.
This is as far as I have gotten with the FSS. I modified the Revell floors to fit the rebuilt FSS and to accept the Gold Modal Models N-scale RR photo-etch stairs and railings.
I have got two sides of the FSS done with the braces. I figure I should have the other sides done in about another month or so. As you can see from the date in the forth photo, this is a long term project.

The way I got it #1

The way I got it #2

After tear down

Sides rebuilt

Modified Revell floors

Basic structure #1

Modified floors

New elevator shaft

Elevator shaft detail

Gold Medal Models 1/160 N-scale p/e rails

Gold Medal Models 1/160 N-scale p/e stairs

GMM stairs #1

GMM stairs #2

GMM stairs and rails #1

GMM stairs and rails #2

GMM stairs and rails #3

GMM stairs done Rails test-fitted to first level

Parts I had to make for the gantry cross braces

First level done

One side half done

Two sides of the gantry done