When my hc started leaking I noticed a spot of fog on the inside of the windshield near the defrost vent and a small trail of Dex coming from the bottom of the heater box (the big black plastic thing that sits behind the console). I layed on the pass. side floorboard and with a flashlight I was able to spot it.
If your hc is leaking then beware. I had minimal warning on mine from the time I spotted the leak to when it completely let go. I noticed my temp was rising quick and my heat went cold. Stopped about a mile from the house and filled the rad back up and by the time I got into my driveway the temp was around 250 and climbing. I was effectively emptying my cooling system onto the floor the longer I drove. Can't imagine that heat being good for the rest of the motor.
And yes, at 131K if the LIM/UIM gaskets are original, then I would be replacing them with the aluminum ones, get some new end gaskets and some gray GM sealant for the corners and call it done. Also get new coolant elbows and a set of fuel injector o-rings. Whether you refill w/ Dex is up to you.
Don't forget to check out your UIM for issues around the EGR pipe. I replaced mine with a new one from ineedparts.com and it included the smaller diameter pipe for the LIM and it also has a metal ring embossed into the UIM where the pipe passes through creating a nice air gap.
