There are two major problems with building a RWD bonneville, one has to do with the car and one has to do with the choice of transaxle.
The bonne rear suspension is not a cradle, the susp bolts to a uni-body structure under the back. All of this will need to be removed to make room for some kind of rear drive assembly. That gone, there are no real frame rails or other structure to build from, so alot of tubing will need to be built. Basically talking about building a "back-half". Not impossible, but very labor intensive. Also involvd will be a tunnel enlargement to make room for the rear prop shaft and exhaust.
The other major obstacle to using the Versatrak transaxle is it is not a true "transfer case" in a 4WD/AWD sense. What it is is basically a Power Take-Off unit (PTO). The front differential in the Verstrak trans looks very much like the diff in any 4T65. However, where the reluctor normally resides, there is a large gear laser welded to the case. This gear rotates with the diff and starts the power path rearward via a series of gears and shafts leading ultimately to an output shaft. This is all direct drive. There is NO CENTER DIFFERENTIAL to allow for front-rear wheel speed bias. in the Versatrak vehicles, the rear differential power is applied via oil pressurized clutch packs (kind of like an auto trans) based on front wheel spin. It is not a true AWD system. It is "on-demand" AWD. And it is not very reliable either. The rear diff assemblies are notorious for failure due to improper towing. Not really the fault of GM, but troublesome for them in reputation. There is alot of electronics involved, making a swap more difficult. Also, the rear carrier assy is very bulky, though nicely designed. It is very tall and would likely jack-up the back of a Bonne, making it really look like a 4WD vehicle!
My idea is to use the VT 4T65 with a center viscous coupler (VC) to allow for some wheel speed variance. The VC would be inline in the driveshaft with one element being part of the front shaft and one element part of the rear. A two-piece driveshaft would be needed, too, with a center bearing for proper driveline alignment. I plan to use a C4 corvette IRS mounted in a custom tubular frame to tie it in with the Grand Prix unibody. The challenge will be to get the gearing correct. The VT 4T65 is a 3.29 final drive with a transfer gear ratio of 1.013:1. That means in a perfect world you would need a rear diff ratio of 3.332. The C4 manual trans cars had a 3.33 option, but it is very rare. Rare, but obtainable. This will work great for straight-ahead driving, such as drag-racing, but as soon as you start to turn, there will be some horrific drivetrain binding. If you've ever been in a 4wd truck in "lock" mode, you know the chatter and squeal of tires and groaning of the driveline. Similar to a drag car with a spool rear end.
1988 Grand Am SE 2.0 Turbo
1996 Bonneville SSEi Twin-Charged