Both trannies are interchangable, however the 4L80 is heavier, longer, and has slightly different gear ratios. The 4L80 is not much like older trannies in that the shifting is all electronic servo controlled. I don't know if gsmikie has dove into these yet. Same principle, but a different animal.
You could always just get a 4l60E from a 1/2ton Chevy 4x4, and use the LT1 converter to get the right stall speed. Then you keep the 4l60's lower first gear and smaller case.
If you plan on towing, etc., you might want the 4l80. Although keep in mind the cam in the LT1 is designed for sports cars, not trucks, so it's going to want higher gears than you might otherwise need.
Lastly, I've been told to stay away from Vette LT1's for swaps. Can't remember the reason off the top of my head, but I know the Camaro / Firebird motor is an easier swap, and it's slightly better tuned than the Impala version. I have a '95 Z-28, and it's a real good motor. Make sure you get at least a '95 to get the optispark version that's improved.
'82 J-10 Laredo, 360, T-177, 4" Rusty's
'85 J-10 (parts)
'09 Pontiac G8 GT Sport, '97 Sierra (Vortec 454), '98 Silverado Z71 (5.7 Vortec)