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!!