One of the cool aspects of discussions like this is that rarely do two people approach a similar situation with exactly the same perspective, which in turn affects the chosen outcome(s). As a result the "right" answer depends on who's approach most matches your own perspective. In the end the broad outcome may be identical (e.g. the car starts, the trans fluid cools) the differences are in the aesthetics, perhaps the chosen chassis layout, the skills/interests of the builder, budget, etc.

papa, I don't have a personal recollection of whether your car is steel or glass, but based on you description of cracks at the belt line just below the quarter windows it must be original steel as that is a very common occurance. When I used the term bulkhead it was loosely as it's structure will vary from car to car. In some it may only be an upholstered piece of panel board, in others it could be a full on structural member of the body. Obviously there are variations in between as well as nothing at all. As you mentioned the original Ford bodies flex quite a bit.............they probably didn't intend for them to last 75 years. It's very common for folks to weld in a tubular structure to add rigidity to the body, especially in roadsters, but also in coupes. The package tray does pretty much nothing to increase body strength, in fact a lot of folks remove it for added space.

A clarification on my comment about the Optima battery. It may not be clear to all that my critique was stated conditionally. I suspect it comes down to how an individual maintains their battery. If you're the type who is very conscious about keeping the battery charge level up, disconnecting or switching off the battery whenever the car is parked for more than a few days or so, and have a good charging system, then the Optima will perform well. Heck, they sell a ton of them so they can't be horrible. My observation is that folks who aren't as meticulous about battery care will let their battery run down requiring a major recharge. A normal lead acid battery doesn't especially like this kind of treatment either, but will recover, especially if it's a deep cycle, marine style unit. Obviously too many of these types of deep cycles won't be tolerated by any battery. That being said, it appears from what I've seen/heard, that the Optima won't tolerate more than a couple/few of these cycles before giving up. They are very good by comparison for high vibration applications. And lots of folks who don't "punish" them probably get good service. They don't offer any particular size advantage, which was the main thrust of my suggestion. Yeah, the Odyssey doesn't recharge quickly, but again tha'ts only going to be an issue if you either have a high electrical demand (ala Dave's window shattering stereo system), or you tend to ignore your battery for long periods of time while it sits idle. Personally the Odyssey works for me because my cars have a relatively light electrical load, and I'm pretty good about hooking my stuff up to a battery tender charger when parked or in a prolonged layover (like winter storage).