I found this on another forum that seems to explain it:
When you are off the clutch in neutral, the input shaft of the trasmission is spinning with the engine, while the output shaft is not, since it's connected to the road. The gears connect the input and output shaft together.
When you push the clutch in and shift into first, the synro has to match the speed of the input shaft (~800rpm) to the speed of the output shaft (0rpm). This can cause the car to lurch forward just a tiny bit, while the syncro slows the input shaft to 0rpm so the gear can fully engage. Since the syncro is connected to the gear, this will put just a tiny bit of torque through to the output shaft. The reason it doesn't happen again if you keep the clutch in, then go to neutral then back to first is because the input and output shaft are already matched.
Have none of you really noticed this? Go out and try it. Start your car, put it in neutral. Then let the clutch out, clutch back in and put it into first gear really fast. The car will lurch forward just a tiny bit. It's normal and isn't a problem.
That pretty well describes what I am feeling in my car, and if that were the only issue I had with the trans I would not worry about it, but as it has been getting worse, and combined with the shifting issues I am having I will have to do something about it this winter.
I tested my 2 other manual trans cars this weekend a 1969 Firebird with a stroked OHC-6 and T-5, and the 77 Astre Formula with a 215 Buick V8 and T-5, they both have 4.11 rear gears, the Firebird weighs 3600 lbs & the Astre weighs 2800 lbs, neither car does it. Nor can I remember any of my other 20 odd manual trans cars or trucks doing it.
After searching for upgrade options and talking to a few "experts" it looks like I will be changing the trans out this winter. Still in the early research stage so I don't have any definite plans as of yet, but Tremec is located close to me and they are willing to look at the car. So that may be the next step. I really do not want to go the 6 speed rought, but it does not look like a TKO 500 or 600 will fit without modifying the trans tunnel and or center chassis. I don't want to go there. So a Magnum 6 speed may be the way to go, but they are $3000 new, and would require a new Bell housing, Clutch, pressure plate, throw out bearing, shifter, rear mount, & drive shaft. That sounds like a easy $5000 bill just in parts. And I haven't priced anything else out yet.
But it is a proven trans that can handle the LS2's torque. And the T56 that it is derived from has been swapped in our cars before so there should be no suprises. I will look at a few other options mentioned as well. Thanks for the ideas, keep them coming. This as all my previous cars mods is starting to snowball. I should be used to it by now.