I've seen your crash photos before Ian, but never the list of your injuries. Dang, you are very lucky to be alive, that is for sure.
Whoever said cars have crush or crumple zones, is absolutely correct. The newer the design, generally the better they are too. (This was invented by Benz many years ago.)
Today, most of the crumple zones are ahead of the front strut towers, and behind the rear wheelhouse area. Cars are basicly three areas of inertia, the front, center and rear, and generally the front has the most inertia due to the engine/trans being there. Our old Supra's are pretty good designs, but they were overbuilt, especially the targa roof models. (This is admitted by the Toyota engineer who was in charge of the car. He admits that it's too heavy due to them not wanting to give up struture for light weight.) This pays off when you crash the car in that there is more to crush, but you pay for it every time you try and do anything else with the car. (A fine trade off in my view.) It also makes the car so quiet and stable/rattle free.
If you are going to "improve" on your cars structure, I reccomend the following. (And I'm also a dead guy here. Died on the operating table for my knee surgery.. still can't sing right because of it... )
If you seam weld, or stitch weld the car, only do it between the strut towers. Leave the front and rear sections alone so they can deform as designed.
Same goes for adding roll cages or support beams.
You can make the passenger area as strong as you want.
As IJ noted, make sure your seat is well attached. You can have the best seat and harness in the world, but if they break loose under a crash, they do you zero good.
Just my .02, but then again, I do deal with this stuff everyday