Wow, this is a contentious group!
All of the following is just my humble opinion.
Let me state up front that, Mr. Kirkham, I LOVE your cars.
To me, the only truly faithful replicas of the original Cobra are the Kirkhams and the aluminum Shelbys. These are the only newly-built "Cobras" that allow the owner virtually to go back to the '60s and buy the same basic machine -- chassis, body, suspension and engine -- that was sold then (aside from the various improvements in materials, etc.). This is VERY desirable -- to me. If I decide that I want the entire experience of the original Cobra without owning an original, I will buy a Kirkham or an aluminum Shelby.
Any non-aluminum body makes it substantially different -- to me. Not necessarily better or worse, or more or less desirable, just different. To me, a 'glass Shelby is too different from an original for me to place greater importance on its being a "real Shelby" than on all the other aspects of the car. The "real Shelby" thing is appealing, but it's not the only factor. Therefore, things such as the chassis design, from a performance and possibly even a crashworthiness viewpoint (absurd as that may seem to some), become considerations -- to me. If I decide to buy a 'glass Cobra, it will be either a Shelby, ERA or SPF based on my evaluation of the entire car (and each company).
Most of what I've read and heard indicates that the original chassis design is really quite excellent from a handling standpoint, remarkably so for something designed so long ago. I would love to see a comparison test among Kirkham/Shelby (alum or 'glass), ERA and SPF since these are the cars on my shortlist. ERA's chassis design is held in high regard by nearly everyone in the Cobra world, and I have yet to hear anyone denigrate it who can support his negative opinion. The criticism usually is restricted to "round frames are better" (the same criticism usually aimed at SPF's chassis). ERA has very clear, engineering-based reasons for designing their chassis with rectangular rails -- this is discussed extensively on their website. They discuss the various advantages and disadvantages of round and square tubes, and explain why they made their choice.
I DON'T KNOW whether ERA's or SPF's chassis are stronger or weaker, stiffer or limper, than each other's or than the original chassis design as seen under Shelbys and Kirkhams. I don't know which is the best handling. I do know that they are all different, and that each will, therefore, have its own performance/handling characteristics.
Crashworthiness:
Well, this is quite the volatile topic...
I have no delusions that ANY Cobra is a great choice of car in which to have a crash. Nevertheless, the wide variance among designs surely means that some will perform better than others in a crash. Even if I cannot expect to walk out of a major high-speed impact in any Cobra, it is quite possible that a not-so-massive crash in Cobra Brand A may result in far more injury to the occupants than that same crash in Cobra Brand B. A different crash may show B to be worse than A. We cannot know whether we will ever have an accident in a Cobra, nor how severe that crash might be. Someone posted that hitting a Suburban in any Cobra will kill you. That's probably true, but if I hit a Civic or a Maxima, I may have a better chance in one Cobra than in another. My point is that some Cobras are likely safer (or less dangerous) than others.
All relatively modern road and race cars are designed to absorb impact forces in a somewhat "controlled" manner. This is, of course, done by designing a "crush zone" into the structure that is designed to deform and absorb the impact rather than sending it straight to the passengers. Cars that have separate chassis frames accomplish this by designing an area (or areas) that will bend when hit hard enough. This is done by having a "kickup" or similar bend in the chassis -- it is at that kickup that the designed-in deformation will occur. Obviously, the chassis is also designed so that the deformation will stop before the cockpit starts to deform. ERA and SPF (among others) have chassis designs that incorporate this kickup. ERA and SPF both discuss crumple zones in their literature.
The original Cobra chassis design was completed well before anyone put much thought into crashworthiness. Whereas the ERA and SPF chassis run the entire length of the cars (bumper to bumper), the original chassis' main tubes run only from the rear axle to just a few inches ahead of the front axle line. Ahead of the main tubes is relatively little structure -- just the much smaller tubes that support the radiator and the body. Behind the main tubes is also a structure of tubes, much smaller than the main rails, that only support the body. In an impact hard enough to crush the car into the main rails, there is no provision for controlling the crash energy, and I have no clue how and where those straight rails will bend.
I do clearly recall reading an article about the AC MkIV when it was new. It stated that the chassis was identical to the original design EXCEPT that a "sheer point" had to be added somewhere in the chassis to allow it to "give" in order to pass the required barrier crash test.
Based on all of this, it seems likely TO ME that an ERA or SPF would better protect its occupants in a front-end or rear-end crash than a Cobra with the original chassis design. The difference may be significant, negligible, or anywhere in between. I could very well be wrong -- perhaps, through pure luck, the original chassis would do better in an impact. Chassis design is one of many factors that affect crashworthiness. Chassis rail thickness, tube size, and material are others. None of these cars look like they'd offer much protection in a side impact. Obviously, the fiberglass used in the body would be a factor, and surely aluminum behaves differently than 'glass. I do believe, however, that the chassis plays a far more significant role in this than does the body material.
The crashworthiness question is by no means my first priority in deciding which Cobra to buy. I know that some companies, such as West Coast/Stallion, place a significant emphasis on this. For various reasons, I have narrowed my choice to ERA, SPF, Kirkham and Shelby ('glass or alum). I don't think that any of these is a "crashworthy" car by any modern standard. I do believe that there may be differences between them (handling and crashworthiness) and that the chassis is something to consider.
Cheers to all.
P.S. Mr. Kirkham, I LOVE your cars.