Funny you should ask this. I've been thinking about this for a few weeks and I'm partially committed. I'm still having fun drag racing my car (when it's not torn apart), but it's missing some fun factor that my mustangs had, a third pedal.
I've been researching what it takes and I think I've got a good plan. Scott Atk is doing a t56 behind a turbo 6, but in a first get camaro, so he has the clutch, bell and trans figured out for you.
http://www.turbobuick.com/forums/threads/bellhousing-flywheel-options-for-t56.379393/
Here is what I'm thinking. These things are all the same for a sbc powered monte carlo, olds powered cutlass or a GN.
The trans is simple. You can buy an LS t56 for $1K+ and throw $2K into a rebuild and upgrades or just buy a new t56 magnum for $3k and have all the upgrades and some that aren't available for an LS trans.
You'll need a quicktime T56 to BOP bellhousing. They offer it for a clutch fork setup of a LS hydrualic throw out bearing.
There are a few options for clutch pedals, but hydraulic seems to be the easiest to source and less prone to issues. I think this one is the best option.
http://www.sickspeedmonte.com/Clutch.html
The length from the back of the block to the trans mount on a t56 is about the same as a th400. You might be able to use one of the th400 conversion trans mounts. Or you can buy a G-force crossmember. Of just cut the old mount of your stock crossmember and fab up a new one. I've already done that for a th400 and it has some adjustment so I'm gambling it will work.
For a driveshaft, the ls/lt an gto t56 are all about .5" longer than a th400. The magnum is about 2" longer than them. So, all will require cutting your driveshaft down. Most of them take a 31 spline yoke.
Where things really change is the clutch and corresponding throw out bearing. So many options and they depend on your goals. Single disc, twin disc, aluminum or steel flywheel, two styles of pressure plate and on and on. Scott Atk got aluminum flywheel twin disc kit made by RAM. I'm still thinking on it, but I'm almost sure I'll buy a billet steel flywheel from TA performance and buy a 1000hp capable RXT Mcleod twin disc kit for it. You want to be careful of high force pressure plate single disc kits. They could put extra strain on the small thrust bearing in a Buick 6, especially on start up when there is no oil pressure.
My billet crank uses the same pilot bearings as a sbc. I imagine the forgings are machined the same as well. The throw out bearing will differ depending on what clutch you buy. I'll have to wait and see, but my friends vette used the stock TOB with his Mcleod twin disc kit and only needed some shimming.
You might get some responses saying it won't work. That seems to be how all the old threads I've found ended up. My combo is not typical (stroked 4.1 and a 67mm turbo). When I get sick of the automatic I will find out.
This is why I'm kinda committed. I scored the bellhousing really cheap and decided to pick up the pedal and master cylinder mount while they are still available.
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