Welcome to Ecomodder. You've set yourself an interesting project.
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Body wise I think I will make the aero back permanent and cut out the back window and move the metal back a foot or more and add cross braces to make the body stiff er.
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You've hit on the reason I wouldn't start with the Rabbit/Caddy myself. What I would do in your place is take off the doors, hang a set of two-door doors, and move the B-pillar and rear bulkhead the rest of your 'foot or more'. Done right it could look factory and solve a problem the extended cab wouldn't.
There's all the other things to do of course (ceramic bearings: marginal gain with some fragility) but to get a real gain right out of the gate: a v1.0 aerolid. If there isn't one for the Rabbit then you get to be first.
Anything you do, you can do in a removable form, at least until you ditch the full width tailgate and boattail the rear of the body. What everyone tries to do is fit The Template over the pickup box as best they can. In thinking about my own case (the aircooled Beetle) I separate it into the narrow, tapered upper body and the wider, rounded lower body and treat them differently. Examples:
If you consider the upper right case as substituting for your pickup, then the others are all possible options. Lower right—truncated Template (no added length), lower left—full boattail (not effective below the beltline and—if you do the math—the full length is brutal but you get to start at the B-pillar, ust like the Beetle) and in upper left is an interesting case: a bubbletop-tonneaued full boattail. That could cover the bottom part of the backlight, if you keep that. It would show nicely, but be pretty ineffective except as a wake filler, which is not nothing.
On the 4-2-1 header, once again you can be first. If you look at a blood vessel or Oak tree branch you'll see the 'primary' bulges before it splits into the 'secondaries' (reversed in exhaust flow). If someone manufactured a header using a hydro-forming process (like they use on Chevy truck and Har-Dav frames) to correctly/scientifically model the interior shape, to fit across the A-1 to A-6 generations, then they'd have an immense market to sell into. I suspect motorcycle exhausts are probably made this way now.