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wondering why you rarely see an ammeter in a car sl6 or otherwise
Ummmmm...because you need a new eyeglasses prescription? 
Most Chrysler products had ammeters for decades until voltmeters began to be phased in starting in the mid-late '80s. All the '60s-'70s cars with slant-6s have an ammeter as factory equipment. Many of the post-'76 ammeters (w/external shunt) don't work very well or last very long, but that's a separate question.Quote:
always see a voltmeter or voltage triggered idiot light
Can't think of many/any slant-6 cars factory equipped with either a voltmeter or a charging system idiot light.Quote:
know that volts are electrical pressure and that a typical gas car needs a charging system putting out 12.5 to 14 volts to keep the battery up
12.5 will drain the battery. A fully-charged "12 volt" automotive battery has an actual voltage of 12.8 volts. The charging voltage must be above 12.8 in order for the battery to be charged. The charging voltage must be higher with colder battery temperatures, because lower battery temperature = increased battery internal resistance. So, line voltage in an actual automobile with the engine running should (and usually will) be between 13.3 and 14.5, depending on system parameters and ambient temperature. GM has traditionally installed voltage regulators with higher-than-appropriate setpoints, and the setpoint has been creeping upward over the years such that some of their vehicles now have engine-running line voltage at or near 15v! This has caused there to be widespread problems with GM vehicles losing brake and tail lamp bulbs so often it was considered a safety hazard by the federal authorities, and GM was forced to fix the problem. Rather than correct the improper voltage, they went through the monkey-motion of speccing and building bulbs rated at 14.0v rather than 12.8v. Talk about solving exhaust smoke by shoving a cork up the tailpipe! 
Typical GM "engineering".Quote:
never saw a post 1956american car with an ammeter
You're simply not looking, then. See above.Quote:
not sure what useful info the ammeter would provide
A working ammeter tells you what's happening with your electrical system on a real-time, dynamic, right-now basis. A voltmeter doesn't, it just tells you the general trend. An ammeter makes it easier to spot a problem early. I don't say that a voltmeter is useless; it provides a different kind of information. But if I have to pick one or the other, my choice is the ammeter. Quote:
i suspect falling amperage would indicate a battery losing or failing tohold its capacity
Not quite. When the ammeter shows "discharge", that indicates more power is coming out of the battery than is being put in. When the ammeter shows "charge", that indicates more power is going into the battery than is being taken out. Either an ammeter or a voltmeter can tell you several different things about what is going on with your electrical system, if the gauge is of decent quality and proper calibration and you know how to read it.