I suggest before you start taking it apart, check the behavior of your shift linkage in ALL gear position,particularly 2nd gear. Not all systems hit all the detents equally. The confusingly named kickdown band is a giant part of 2nd gear.We are asking this band to stop and then hold a heavy front drum under full power.Much bigger job than the low band which just needs to hold from a much lower speed. An aluminum front drum, being lighter, should theoretically help in this situation.I described fighting this situation in another post involving clutchflites.I learned about the importance of keeping an eye on the kickdown band adjustment, especially in a heavier and/or higher horsepower car. Not at all a difficult task.
With an assistant (easier,but not required) move shifter one postion at a time and individually check lever for full engagement in each position, meaning shift cable pivot can be moved in and out of lever with no side load. Just checking in Park or Neutral is not enough. Both the shifter lever and manual lever move in an arc, so you can see where variations could be evident. I failed to do this after a rebuild,and on two weekends had a trans that went from 1st to 3rd,skipping right by 2nd under full load. Didnt see it on burnout or light load either. Thought problem was internal caused by my rebuild.Thought about linkage and did the test I just described.There it was.Revamped linkage a bit to ensure lever was fully engaged in 2nd gear and trans performed correctly. No more problems and kept an eye on kickdown band adjustment.Good luck Bill