My point is that no one ever fit in the Spyder with their noggin below the top of the windscreen.
Unless the windscreen was raised.
Which was done, on occasion, in period.
As you say, you can't take the floor down more than an inch or so without making real trouble for yourself. You can't make the seats lower than "bolt the shells directly to the floor," as you have done.
I think what @Stan Galat wants is not a 550 Spyder kit or replica, but a reimagined Spyderesque vehicle at approximately 1:1.1 scale, so that he (or anyone) could sit down "in" the car completely and yet retain the full measure of that iconic super-low profile. I'm sure it's doable, at a price point. It might even be worth doing, given the changing market and modern expectations of what a sports car ought to do and be and look like with a person driving in it.
I know Stan dislikes looking like (in his inimitable phrase) "a circus bear riding a tricycle." Seems so undignified! So counter to the idea of the sleek new Porsche upending the racing world.
BUT: I have checked, and BEAR-ON-CHILD'S-TRIKE is exactly how every legendary driver looked in these cars, all the time, when they were raced.