Most people running a narrowed beam in a spyder only do 1" per side, aka 2" narrowed beam. I've also had custom beams made that were only 1/2" per side, 1" total. They do add a bit of bump steer, but as Danny said it is pretty much negligible. You don't feel it in a really negative way until you get into the much more narrow beams, which are also usually coupled with much wider tires, exaggerating the feel in the light front end.
When deciding if this is right for you, consider your wheels also. The Spyder runs a 4.5" wide front and the steel wheels are a 45mm offset, while the aluminum wheels are a 32mm offset. Thus, the main reason most run a narrowed front beam is simply to use the alloy fronts and not have rubbing issues on th router wheel well lip, or to use one of the upgraded brake kits that add considerable offset and not have the same aforementioned rubbing issues. There are a lot of combinations you can run in the front, just keep in mind that changes have other effects that you may not consider. Reduced turning radius, ineffective steering stops, increased rubbing on the front of the wheel wells, to name a few. Nothing crazy, and nothing that can't be overcome, just something to keep in the back of your mind as design your build.
I've never seen a Spyder pulled out of a body mold prior to receiving chassis and internal panels, but in your case that may be somewhat of a benefit as you can build and/or reshape your internal panels to match your needs. The front of the front wheel wells is pretty easy to reshape with little adverse effects. Reshaping the rear of the front wheel wells immediately starts to get into the chassis and pedal box area, which is tight already.
Interested to see what you come up with for rear suspension design. Many have tried, most have failed, although I've seen (and tested) a few designs that worked well, but we defer to swingaxle and likely always will at this point.
I don't know your chassis specifics, other than what I can see in the photos, but it appears to be a copy of our early chassis with a change to how the front beam is attached and a change to how the rear torsion bar is braced. Assuming it is a 0.120 wall DOM tube and that the aforementioned changes do not change chassis stiffness, you should have no issue with ADR for the torsional tests. Subsequent to your previous conversation with Chuck with actually did go through ADR approval on our watercooled inline 4 chassis, and in the end wound up with an ADR importation number for that particular product. The main chassis structure was the same as aircooled, but when we discontinued that particular drivetrain, the napproval was lost and not applicable to Subaru or aircooled. Anyway the issues they had, at the time at least, were with lack of headrests, 3 point seat belt mounting, seat shell construction, and the lack off steel reinforcement inside the doors. Chassis rigidity was not an issue.
Side note, that C&D article was a total fluke, last minute, and simply because Chuck was friendly with the magazine and close by when they needed something. After a days worth of shooting pics it had turned from "maybe we'll mention you" to the cover shot. A poster of that cover still hangs in the shop today.