I've seen builds done both ways, converting from a Model S drive unit. There's an Audi RS5 conversion with the Quattro drive system in-tact because the builder mated the Tesla motor and inverter in the engine bay and hooked it directly into the Audi transmission.
As much as I love some of the ultra widebody early Z builds, as you picked up on, I'm in Germany, and I'd love to be able to use this car on the road, so things like reengineering sub frames may be out of the question, but keeping "stock" (or at least e-marked and certified) suspension, subframes, etc, etc may allow me to restore the car as a motor-swapped restoration vs. an entirely custom build, if I deviate too far from the original designs then I could be looking at 7-10k EUR in costs to have a special inspection performed and it'd be counted as a one-of-a-kind build and essentially treated as-if I'd done a clean-room build from sheet metal in my garage.
For what it's worth the track width of the Z is 53" (1346mm) 66" (1699mm) - but I think that the TÜV (vehicle inspection authority) will look unfavourably on serious mechanical modifications.
Whatever I pick-up will likely be rusted to shit, so I'm thinking of building a full cage, and bracing the frame whilst I cut out the floor and sills. That would have given me the option to do-away with the tunnel, and modify some of the floor pan to take batteries, but sends me firmly down the "one-of-a-kind" route in the eyes of the TÜV. Alternatively with a forward mounted motor directly onto a modified drive shaft leaves plenty of space in the engine bay for a few batteries, not to mention the former home of the fuel tank and the spare wheel well.
The complete Tesla Model S battery pack with 85kWh weighs a stonking 1200lb (540kg) but is modular, comprising 16 modules each giving about 5kWh and weighing in at 55lbs (24kg). They're fairly manageable sized too, 26.2×11.9x3.1" (78×302×665mm). Given the weight it seems wise of Tesla to have mounted it in the floor!
I'm yet to look up weight distribution figures on a regular 240Z, but it shouldn't be toooo difficult to fathom where the centre of gravity would fall if I went with say, 50kWh of batteries (at 550lb/240kg) (which would run me about 10×1200EUR for the packs, and give me something like 180 miles of range) and the Tesla drive unit (295lb/133kg) which can be sourced for 5-8k EUR plus 2-4k in ancillaries (controllers, etc).
It's perfectly plausible to split up the battery packs, they need high voltage cable, and coolant running to them, but that's the most basic of basic plumbing. I expect the plumbing and wiring to be easier than dealing with the plumbing and wiring for an ICE.
The money aspect felt quite interesting to me, I've followed the Mighty Car Mods 240Z build (well they think it's a 240!) where they motorswapped for an RB26, but sourcing those motors in Europe now also begins at 15k, and that's "original" status, where they still need a modern turbo upgrade, upgrades to proper modern fuel injection, modern coils, and electronic management, new exhaust manufacture, etc, etc, etc.
Even with higher than average costs to get a permit to use it on the street here, I can imagine that a Tesla conversion and a modernized RB26 upgrade would come in at roughly the same price. Starting costs for the drive train alone are 15k EUR with a 5k "adaptation" budget.
To touch on the CAD aspect momentarily, I'm a software engineer by trade, so the idea of managing a build and designing my own parts requirements comes more naturally to me than sitting with a sawzall and hacking at it. I can weld, but not excellently, and certainly never tried to weld Aluminium, which is a skill I'd need to pick up.
Land prices are so high in my home city that if I want to rent access to a shed with a bay and some tools it'll easily add 500EUR/month to my costs for this car, which pushes it almost to the point where I can't justify the cost. At least I can't justify dragging the build out over a year, and being 6k EUR out of pocket for having had the luxury of taking my time.
So my "plan" (in so far as I have one) was to find a (complete) rolling shell, get it stripped and start bodywork and chassis changes on it, but try and get it back to stock.
Get it dipped to strip the old paint and misc and then anodized and protected. From there I wanted to get the chassis and fenders/etc scanned with hand held 3d scanner, I think a clean scan of a 240Z chassis would be a valuable resource for the community in general, and would allow me to plan the rest of the build virtually whilst getting all the parts made up.
Hand-in-hand this approach may actually help me keep costs and timeline short, but it depends on my abilities in software more than my greasemonkey skills...
The whole thing may turn out to be prohibitively expensive in Germany, right now a rusted out shell in piss-poor condition is selling for 6-8k EUR (for e.g this one).
I was hoping to get close to running and driving for circa 30k EUR within 1-1.5 calendar years, but with a 3 month active build time of real workshop time.