I was about to start a similar thread when i saw this one. Hate to hijack, but my question partially answers his, so i didn't think the OP would mind.

I got a 99 stock GTP, need a transmission rebuild.

To the OP, I think you're gonna be in it for a total rebuild to make it truly stronger. I'm going to try to rebuild mine myself, and also looking for ways to make it better without going north of 8 grand like Adecker. Here's what I've found so far, hopefully someone else can provide more direction.

These transmissions were updated a few times through the years, including electronics and hard parts. The electronics are not interchangeable, but the improved hard parts are. ( I think) Early models commonly had failures resulting from stripping the splines off the 4th clutch hub. In either 2003 or 2004, GM solved the problem by using one with heat treated, hardened splines. Also, the reverse reaction carrier was changed a few times as well. Some of them have rectangular or oval shaped holes in them, and others have circular holes and the castle top part is heat treated and hardened. I don't know what years though. I think there were more updated parts than those two, but I'm not totally sure yet.

My hope was to identify more of these parts that were changed through the years, figure out what years the changes occurred, and pull one from a junk yard that, hopefully, has them all. Likely from an 04+. This way, I can get some practice in taking it apart and putting it back together before I try it on mine, and hopefully salvage some of the better hard parts and swap them into mine, resulting in a better transmission. Does anyone have more info on this?

Even though it doesn't make the transmission truly stronger, I've read this about modifying it in search of quicker shifts, which would add longer life. Of course there are shift kits, TransGo seems to be the most popular, although Superior makes one also that is said to be very good. I've read you can insert washers into one or more of the accumulators to improve shifts, enlarging some of the holes in the valve body spacer will help improve shifts also if done correctly. Those things can be done without removing the channel plate, which makes them "easy" I guess.

Using oversized steels in some of the clutch packs also helps improve shift feel and speed, but by time you get to that, you're in there and it's pretty much overhaul time. Lastly, according to the W-body tech page at the Overkill motorsports site, if you have a GTP, you can swap from a 258mm torque converter to the smaller and lighter 245mm torque converter. Will claims in his experience that this will put less weight on the input shaft, and therefore make it last longer and with careful selection of the new converter a performance gain can be had. To do that, you also need the flex plate from a N/A 3800 as well he warns.

If my info is wrong, someone please correct it.