The carrier is now complete. I left off a series of little chain-link guardrails on the upper deck that wouldn't stop you from falling. I also mounted the forward upper deck section much higher than the instructions called for; the way it was set up, there was only room for one car at the front for a total of 5. At this point it will hold 6. (The yellow body on the front upper deck is the diminutive Honda S800).
The lower deck falls between the wheel arches and is pretty narrow; the RX7 body won't fit because its wheel arches are too wide. The orange Skyline 2000 fits up to the rear wheel arches, which are also too wide. The overall width of the trailer works out to 102" at 1:1, so even with the single wheels, there isn't a lot of room. Furthermore, the rearmost lower deck position is such that I am not sure that you could actually get out of the car, even if it did fit.
I see there is an upcoming re-release of the Revell auto transport listed on the Model Roundup page (http://www.modelroundup.com/product-p/rmx-1509.htm). Presumably the overall width is similar, but the lower ramp rises above the dual wheel sets so that much wider cars (such as, for instance, 1959 Impalas) might fit on the lower deck.
My existing cabinet will just take the trailer and six cars, plus three more short ones in front of it at 'street' level, if I take out a shelf that used to hold at least 6 cars. Plus the Mitsubishi tractor doesn't fit. So I am replacing space for at least twelve cars with a trailer and at most nine cars. One step forward, two back ... I think I'll need to build or look for a bigger cabinet as there is an incomplete White Freightliner on the shelf, which will need space, especially if I build it with the new Revell auto carrier ... Not to mention the stash which includes a Bedford OLBD 5-ton dropside, a Bussing 8000 S13, a Krupp Titan SWL 80, a Ford LN8000 car carrier and a Dodge A600 cab with Little Red Wagon on a trailer, all waiting to be started. We're looking at a whole lot of mission creep here.