For what it is worth... i had the original lt1 4L60e until the torque converter weight came off and split my bellhouse so i switched to the ls1 split bellhouse 4L60E for a while but i didn't like the way it fit and looked under there... i also thought it felt heavier so i decided to go back to the lt1 4L60E...

In doing so i decided to way both transmissions without the torque converter and found out that the ls1 4L60E weights 148.5 lbs and the lt1 weights 128.0 lbs... 20 lbs less...
It seems that where they saved weight with the aluminum block they then added weight with the transmission...
Just some food for thought...

 

are you sure it wasn't something like one was drained and the other wasn't? the differences between the two are mostly external. all the internals are the same size and weight. the one with the removable bell might weight a little more because the bell completely encloses the converter and the added weight of the bolts to hold it on but those two things don't equal 20 lbs.

 

I thought the LS1 and LT1 blocks had different bolt patterns on the back for the transmission. I didn't think you could swap the two.

 

both were empty and niether had the dipstick in them and the ls1 4l60e also has a slightly longer (or shorter, $hit i don't remember now) tailhousing...
i'm just tell you all what my scale read...
i bet the ls1's oil pan weights more also but they have a composite intake to make up for that...

EDIT: sorry for my inaccurate post but i just remembered that the ls1 4l60e was weighted with the crossmember still attached which might weight 3-4 lbs...

same bolt pattern but the ls1's oil pan accepts the bolts for the bottom of the bellhousing... tailshafts are different...