I removed the shells from my early Atlas RS-1's so I could install an ERR Cruise Commander. It was such a pain that I wrote down the procedure. Here it is.
You will need a precision set of Phillips screwdrivers, especially one with
a long and very thin shaft. The shaft needs to fit between the drive wheel
and the sideframe of the trucks. If you don't have a screwdriver this
skinny, don't even try to remove the shell! It is also very helpful if your
screwdriver is magnetized so you can lift out the screws. This is more
important when re-installing the shell, since there is very little room to
work when trying to put the screws back in!
The Atlas instructions show the "general" location of the screws that hold
the shell on. There are 6 of them: 2 at each end of the hood, and 2 where
the long hood meets the cab. These last 2 are easy to get out. The ones at
the other end of the long hood are not too bad, they are a little hidden
under the truck but the truck can be swiveled enough to get a small Phillips
screwdriver on them. The ones at the end of the short hood are the biggest
problem. They are way under the truck and you have to use the thin
screwdriver inserted between the truck sideframe and the wheel (with the
truck properly rotated) in order to remove the screws.
Once all 6 screws are out, you can remove the shell. There are actually two
shells: the short hood and cab are one assembly and the long hood is a
separate piece. Remove the short hood/cab first. The cab has 2 plastic tabs
that fit into slots in the walkway at the long hood end. There are no
locking tabs, but be aware of these tabs, especially when re-assembling. The
handrails fit into small holes in the cab, and the low handrails will keep
the cab from coming off. Grab the handrail wire near the steps on the pilot
and you can slide the rail out of the hole in the cab (it will slide through
all of the stanchions). You'll need to release the rail on both the short
and long hood. The high handrails can be flexed easily out of the holes near
the cab roof.
With the handrails released, the cab should start to lift. I used a small
flat-blade screwdriver to help get it started. The cab will lift off, but
the wires to the headlight LED are just barely long enough, so the cab/short
hood piece cannot really be set aside.
To remove the long hood, the plastic bosses that house the screw holes must
be flexed apart so they can slip past the smoke unit. These bosses are at
the cab end and you can lift the hood enough to get your fingers in there to
gently spread them apart.