LS1 engines are definitely not "more available", than their cast-iron counterpart 4.8L/5.3L/6.0L. I don't have any production #s, but I'm pretty sure there were more trucks/suvs produced than vettes and f-bodies. Just guessing.
Yes, the 4.8, 5.3 and 6.0 "Vortec" truck/suv engines are "LS based".
Gen III / Gen IV is the broad term to refer to the "LS based" engines.
Some items to consider:
-I think I saw some g-body Gen III / IV swappers using f-body manifolds? Or was it the stock truck ones?
-There's a few companies out there that can reprogram the stock ECM and sort your wiring for you Speartech and Wait4Me come to mind.
-As others have mentioned, if it's a DBW (drive by wire) throttle body you'll need the pedal and associated wiring to adapt into your g-body.
-The truck oilpan will hang below your engine crossmember, depending on how low your car is, this may be a hazard.)
-I believe a cadillac cts-v pan, or a modified f-body pan is considered to be the cats ass for g-body swaps.
-If you run A/C you will have to relocate the A/C compressor, or notch your frame to fit.
-High pressure supply, regulated to (I believe) 58psi.
-Hardlines, hose, fittings
-Many guys use the regulator from a corvette which is a fuel pressure regulator and filter in one. Google "Corvette FPR" and you will find loads of info. Or google one of the following:
GM# 10299146, AC Delco #GF822, Napa# 3737, Wix# 33737
if you're planning on throwing your car around any corners with the level of the tank low, you'll need some way of making sure your fuel pump inlet stays flooded. Baffles, a surge tank, or a fuel module from a newer vehicle. Adapting a newer fuel module into your g-body tank may allow you to skip the corvette FPR, since some have internal regulation.
I see it's going to be a cruiser... so you'd probably be fine, just don't try taking any long, sharp corners with small amounts of fuel in your tank.)
I believe that I saw a guy with an El Camino on LS1tech.com saying that he fit the truck accessories under his hood...?
Not sure how the engine compartment compares between elcaminos and cutlasses. I imagine it'll be pretty similar.
A REALLY GOOD source of information is... wait for it... http://www.LS1tech.com
There's a few threads over there on swapping into g-bodies, some with pics. I remember there was one over on maliburacing.com too.