My 2 cents on Lean Cruise,
Take all this as not absolute fact, majority is not from my own personal experiance, but from hours and hours of reading things on the internet over the several years.

Lambda 1 stoichiometric 14.7 : 1 100% gas is the mixture that produces the most heat. Less fuel, less heat, More fuel, the fuel has a cooling effect to some degree.

Usually when people talk about running lean and doing damage, as far as I understand, it is mostly during a load that would require power enrichment and the cooling effect from the extra fuel, and possibly detonation. This is where the Cruise part of Lean Cruise comes in. During cruise conditions, there shouldn't be enough load to cause any damage to anything, assuming you have no knock.

People who talk about setting up carburetors for lean cruise, usually say lean it out until you get a little lean stubble, then either add a degree or two of timing, or go just a bit richer to eliminate the stumble. Every combination of vehicle and engine will want something a little different. Most LS engines I have read about, people seem to say that to lean negates any MPG benefit, and actuality uses more than leaving it stock. They usually don't say at what KPA they go from rich to lean though.

On flat ground, maintaining speed what KPA do you cruise at. What KPA does it idle at. If you are at 80 kpa most of the time, you probably wont see much improvement.
But you should be able to go fairly lean until about 70 or 75 kpa. If you look at a stock $OD bin on the Open loop AFR table, you can see that they have low load areas lean until around there. Of course it would never use that stock in closed loop, but gives you an idea of what GM thought was safe in the 90's. I think they have it set to around 16, or 16.5, or so. On your LS, I would take advantage of the spark adder table that adds or subtracts timing for deviations from stoich.
Be careful as I think most bins are set up for vacuum kpa, and not Manifold Absolute pressure. Big difference

Stock on a truck tune, I think it would be debatable how much damage you could cause running lean at high load, with the theory of less fuel is less heat, being that a stock truck tune usually doesn't take full advantage of Power enrichment. It is usually on a delayed timer and ramped in at a slow rate. I had a 2006 Dodge 1500, after looking at the tune in it, I seen it was set up to never engage PE unless It was floored TPS 100%. I pulled trailers to heavy for a half ton lots and lots of miles. It has close to 300K miles on it and it never melted anything. If the most heat is at stoich, this should have killed that engine if running lean would.

As far as how lean is safe, from what I know, leaner is safer, as long as there is no heat, or knock, detonation.

I've look at the holden stock lean cruise bits before, and thought they were a little conservative, I'm surprised you were getting 17:1 commanded with it, but its been a while since I looked at it. Did you notice any MPG improvement? Leaner doesn't always mean better mpg, because you loose a little power, and that increases load.
googling shows best Fuel economy at 15.8 - 17, depending on what chart you find.

If anyone wants to correct me on any of that, please do. I remember how hard it was to find anything on lean cruise when I would look the last couple of years.
I have gained so much from this forum I would like to be able to give some back. Hope that helps