From the little bit of research I've done its definitely not a bolt in swap. Gotta break out the welder to make your own engine mounts. The manual trans on a NA Z31 is the same used for SR20 and KA so you might not have to change the driveshaft length but I couldn't find a definitive answer on this. Another thing to consider is the engine management side of a swap like this. You're going to want some sort of piggyback tuner or stand alone ECM. Going with one can make wiring the swap much simpler as you can ditch things like evap and generally make the swap neater and quicker.

The Z31's are cool looking especially the last ones. But they're heavier than a s13 and there's way more documentation for s13 sr20 swaps and putting a turbo on a KA24. Sure you'll spend a bit more initially for a decent s13 but you'll save time, have a lighter car, and potentially spend less on the swap. The trans may be the same between z31 and s13 but the driveshaft may still need to be modified, may need a different bell housing, then there's the engine mounts, etc, etc.

Trust me when I say project cars always cost more and take way longer than anticipated. My 1967 Camaro project is going on 5 years now. Sure it stalled because of my new house and other things, but even while I was working on it I ran into unexpected things that added months more time to the project. Going with something more documented can shave lots of time and cost. Z31 V8 and VG30det swaps are way more documented for example.

To me Flexfuel means the vehicle automatically detects the fuel (usually either pump gas or E85) being run and adjusts the ignition timing, injector pulse width and boost accordingly. At least that's what factory "Flexfuel" vehicles do. The alternative is to manually load the E85, race or pump gas map into the ecm when you fill up the tank with a fuel that's different from the last time you filled up.

For those that don't k is E85 is 85% ethanol 15% gas and has a higher octane than pump gas. So you can run more boost and ignition timing for more power. But E85 also requires more of it to be injected into the engine than gas so larger injectors and very different injector maps are needed in the ecm. In short you need to tune each separately and have the proper map loaded in the ecm for the fuel being used. Switching from gas to E85? Gotta switch maps somehow.

I'm curious as to what he's running for engine management and how the flex fuel capability would be added.