LS1's shed at least 100 lbs., or for a Gen1: aluminum heads, water pump, intake. You could lose the heater, AC, power locks/windows (if you got 'em), use a lightweight battery, and welding in Mustang II style front suspension with tubular A-arms and coilovers will definitely shed some weight, though you would probably lose some strength (not a problem if its not a work truck). Oh and another thing, how far do you really need to go between fill-ups? A 15 gal fuel cell will save you over 100 lbs vs. a pair of 16 gal. saddle tanks, when full. Or you could just not fill your tanks up all the way, but this is a good excuse for ditching those lousy saddle tanks.
If it were me, I would switch to the fiberglass front clip, but not the cab or bed. It's nice to be able to weld in a roll cage (if your plans involve going fast you gotta consider the added likelihood of losing control and crashing). And you don't really want to lose any more weight over the rear wheels because this is counterproductive to putting power to the ground. Just my $0.02