I fitted an Ami Super chassis and engine in an Acadiane a few years ago. It's quite a lot of work, but when I did it I had a collapsed chassis on the Acad and happened to have a spare rolling Super chassis with engine and box, so it seemed like the obvious thing to do. There are a few things that you need to be aware of before you start.
As other people have said, the G-type engine and gearbox will fit in a Dyane/Acadiane (not the 2CV) without the need to extend the bodywork, but only if you use the shorter bell-housing and input shaft from the Ami Super. This gives enough room for the 4-speed gearbox to fit between the engine and the steering rack. There is not room for the 5-speed box. I have heard of people tilting the engine/gearbox up at the back so that a 5-speed box fits above the rack (with a hole cut in the bulk-head. This tilt can cause oil circulation/starvation problems in the engine. I have also heard of people trying to get around this by raising the engine too, so that it's level, but that's not good either: The engine is much heavier than the 2CV/Dyane engine and cornering* suffers as a result, so raising all that the weight will make this even worse. (*Cornering: even with the front and rear anti-roll bars that were standard on the Ami Super, the handling/cornering is not as predictable as the 2CV: When cornering enthusiastically, as you get to the apex of the corner, you have to turn more as the weight of the engine in front of the wheels, causes understeer). You also need to use the Ami Super engine mountings, or you can modify the GS ones. If you use standard GS ones, they foul the brake discs. The GS gearbox has a very short lever coming out of the top, so you need to make something up to extend it, to attach the lever that goes through the bulk-head. Then you need to get used to the different gear positions: 1st and 3rd are in the same place as on a 2CV/Dyane/Acadiane, 2nd and forth are where 2CVs have reverse and 2nd. So to go from first to second, (or from 3rd to 4th) you move the lever straight forward.
I would recommend that you fit the front anti-roll bar and mounting plates from an Ami 8 or Ami Super, which is simple. Don't be tempted to use the Ami steering rack, as the angle of the pinion is different. You can not use the Ami super rear anti-roll bar: The plate that the knife-edge locates in is shaped on an Acadiane, to clear the cross-member under the Acadiane rear floor. Even if you drill holes in it to mount the anti-roll bar clamping brackets, there still isn't room for the bar. You also can't use the Ami Super arms, as apart from being closer together (the track is narrower on an Ami than on other A-series), you will also find that the knife-edge plates will fowl things, are too short, and the levering angle is wrong.
The Acadiane chassis and wheel-base is longer than any other A-series, including the Ami Super: If the Acadiane has got it's original chassis, you will be able to see where it has been extended at the back: the spot welds can be drilled out and the extensions removed and fitted to the Ami Super chassis. Similarly, you will be able to see where the positions of the reinforcing bracket etc for the rear axle tube bolts differ on the two chassis: these will need removing in a similar way, and welding in securely on the Ami Super chassis.
If you use the Ami Super (not Ami 8) front bumper brackets, (that bolt to the side of the Ami Super chassis), you can mount the front panel directly to them, as there isn't room for the standard Dyane brackets, etc. This makes it easier and quicker than to dismantle the front end than on a standard Dyane.
There's no room for a bonnet catch, so I used metal clasp things on each side. There was room under the bonnet for a 2CV plastic air filter box on the right-hand side, attached to the carb with a combination of the original GS/Ami-Super rubber elbow and a short (straight) section of 2CV rubber elbow inside it. I used the Ami Super heater fan. There was still enough room for the spare wheel under the bonnet.
Once it's all done, and you've got used to the gear positions, it's great fun to drive. Using the standard Ami Super gearbox ratios, I reckon it did close to 100mph. Strangely it had a slightly higher top speed than my other car at the time: Ami Super with 1.3 GSA engine and G-Special high-4th-ratio gearbox). Both had appalling fuel consumption, but that's probably more down to me.
I sold my Acadiane about 6-7 years ago, just after the Kelso world meeting. If anyone knows where it is now, please let me know: PMT120X. It was dark green with an orange front panel. The last I heard of it, it was somewhere around Portsmouth about 4 years ago, having an exhaust and some welding for it's mot. I had used the Ami-Super exhaust kidney-box and swan-neck, (which are both difficult to find now), with a standard side topedo (which is what the Ami Super had anyway) and short (van/mehari) tailpipe.
Jeroen Cats (sadly deceased) had a really good website for more info on similar and other modifications, lists of gear ratios, etc:
http://www.cats-citroen.net/" onclick="window.open(this.href);return false;
