Thanks, everyone for the great suggestions. After reading through the replies, I think I may end up taking TF360's advice, to just leave the side drain plug in place and keep backflushing it as I have in the past. I have a bad feeling about trying to get that plug out after it has been in there for the past 34 years. Usually, I just foul it up somehow and end up opening that can of worms that he mentions. Toolmanmike asked why I feel the need to remove every drop of dirty coolant from the system, which is a good question. There's no logical reason, other than I'm a bit obsessive about purging old coolant. Since 2019, I've replaced the incorrect radiator that the car came with and installed a correct part number recored and rebuilt radiator. I've also replaced the water pump, thermostat and housing, heater core, all heater and radiator hoses, and installed a NOS correct part number (one year only for 1974) radiator cap. There's nothing left to replace in the system, and it cools beautifully. Thanks again to all who took the time to weigh in with their opinions. You guys have been extraordinarily helpful in keeping my 49 year old car running like a brand new Plymouth. Muchos gracias!
