Scott, I'm gonna weigh in here, maybe not my place.
1st wish you were my dad when I wanted to run off on some crazy hair brained scheme, I had to learn from his mistakes, not mine, so I got to make the mistakes later in life.
If you don't want to junk up the dash with an aftermarket heat gauge that has actual temps on it, buy one of those laser pointer infra red thermometers and get the real temperature at the thermostat housing.
Blowing a head gasket at 160 degrees doesn't make sense. I tend to agree with Brian, fix the head gasket and put in a 160, but if your radiator is clogged or you have a blocked cooling package, it won't fix the problem. Rather than loosening the rad cap or getting a lower pressure one, you can drill a _small_ hole in the thermostat to let more flow through.
For the minimal cost of getting that cooling system flushed and radiator done correctly, I'd do it before going to Mexico. Not to sound xenophobic, but with the stories of young Americans disappearing, wouldn't it be best to arm your child with a vehicle that is reliable when traveling across a foreign (and yes Mexico is foreign, did it on a motorcycle and a vw bus) country.
Sorry to get off topic and rant.
Chris
