Baz i cant imagine how you are feeling tonight!
Ok... calm breaths.
If thats all it is, i dont see it as that bad. Welding a subframe in is pretty simple, the only problem is it stuffs the paint on the chassis (ok thats a big problem if its as nice as yours).
With the demister, is there any reason why you cant run some flexible rubber pipe from the vents to the windscreen? As long as it doesnt obstruct the gauges or controls i dont see how they can knock it back. Could you imagine the look on their faces!
and easy to undo.
How was the steering rack shortened? With mine they just cut down the shaft where the tie rod screws on and rethreaded it (so i was told). If thats the case its a simple visual inspection and measurement of the height of the thread.
Keep us up dated.