According to Sallee Chevrolet, here's some info on the 4L60E:
The 4L60 transmission is the successor to the workhorse 700R4 four-speed automatic.... Weight (with converter): 164 lbs. (dry) 184 lbs. (wet) ...
Using that info, if you subtract the trans weight from the overall weight, it puts the LSX (with both motors not having A/C) at 425 lbs (wet) and the L24 at 365ish (if you add oil & oil pump), so a 60 lbs difference. Not too bad for triple the HP!!