Need a Mopar guru: 273 part-throttle ping driving me crazy!

Usually a 50/50 mix is good on coolant unless you get down to 20 below or more. Just check it with an antifreeze tester. If it tests to -40 or so you are probably on the overkill side. Mopar had issues with what you are on some 80s model Carter BBD series carbs and hooked up an adjustable metering rod linkage to over ride the vacuum so that they wouldn't get pulled down at part throttle and cause your problem. Usually the T-stat is marked somewhere.