Because it may cause an old engine to leak. And it is not recommended for may gearset applications like rear gears, some manual trans applications etc.
If you don't know anyone that does tranny's you could just swap it out for a 350 that has already been rebuilt. Or you could opt for a 700R4. Jeg's has a plate that will allow it to bolt to a BOP pattern. That would give you an extra low first gear and then an overdrive at the top. Might be a lot easier than finding everything to put a 4 speed in.
First off, I would not loose sleep over the clunk in the trans... it will most likely last 10 more years. If you do want an overdrive trans then go for a 200r4 however you will want to change the rear gear to match the overdrive. if you were to use an overdrive trans with your highway gear you would find it was only happy over 75 mph on the highway. If you like driving 80+ mph then keep the highway gear and add the overdrive. The 200r4 and 700r4 are non computer trans, no issues there you can run either trans with your Buick 350 no major issues. The 200r4 is the best swap for you since it is the same length as your th350. The 700r4 requires a new drive shaft, an adapter plate and a few other hassles. There are many write ups on how to make this work... Some info here: http://www.v8buick.com/showthread.php?t=109774&highlight=overdrive+swap
