If we plate only the outside, you end up with an effective section modulus of 1.35in^3, still considerably more stiff than the original frame. What also needs to be considered are some of the torsional effects from the motor. Any plate added anywhere, will add some torsional rigidity. What is good about the plate on the side, it adds more torsional rigidity than the plates top and bottom. Which brings up another issue, crossmembers. With an SAS we are fighting differential clearance, and this does not make adding crossmembers easy. Personally, I think it would be beneficial to add a crossmember to the frame, once the IFS is cut out. That being said, this crossmember could be rather small. Something as small as a 1.75" OD tube would do quite nicely. A frame plated on the outside only, with a crossmember right in front of the sump, tucked as tightly as possible, would be great. I have not done an SAS, so I can't say what clearances are there, and what are not. For me personally, I am an advocate of low lifts so the center of gravity is as low as possible. I would like to SAS my Trooper, but I only want to run a 33x9.50 tire. I really only want a few inches of lift, so I know that differential clearance will be at a premium on my rig. What crossmembers are being kept and what crossmembers are being discarded on most of the Isuzu SAS's?
By the way, if you can't tell, I get almost giddy when I can apply my vocation to my avocation <img src="/forums/images/graemlins/smile.gif" alt="" />! I am a geek by profession, and it is exciting when I can apply schooling to real world stuff. Not that I don't do that at work, but frankly, I design and build boring stuff at work. If I wasn't such a chicken, I might venture out on my own and design things I like. But I am afraid if I did that I would not be able to adequately provide for my family. There is a lot of security in a benefits package! Dan, sorry for hijacking your post, but I think this can benefit anyone considering an SAS.
That being said, a plate on the side is probably quite adequate. If you want to overkill it a little, add a crossmember, and if you really want to overkill it, add straps to the top and bottom.
One other thing, any plates you add top, bottom, or sides, it is a good idea to taper the ends so you don't have an abrupt change in cross section. Abrupt changes create stress concentrations, which can lead to premature failure.