Look on the driving shell (behind the front drum) and there will be witness marks where the drum tangs have worn on the shell...this will tell you if the drum in installed deep enough.

The output shaft ball bearing snap ring will usually have some movement in the tailhousing, resting the weight of the trans on the output shaft will shove the output shaft as far forward as possible and can thus skew the end play reading. Some builders advocate doing this on purpose to load the output shaft full forward, I prefer to take the end play reading with the trans horizontal.

Keep in mind that there are two selective thickness fiber thrust washers, even with the thinnest one behind the pump the other one might be thick enough to tighten the end play too much.