My modified 260 was timed, didn’t use radar at the time, at 147 in the early 70’s at MIS (Michigan). It was before air dams. Car was down right dangerous and unstable. Scared to death when getting out of car. Shook for an hour.
Re, flushing. General flushing ,even with chemicals only removes the loose stuff. Therefore, if there are contaminantes, rust and compacted sedimate residing in passages etc, they will continue to be dislodged through general use. And worse, accumulactions will remain lodged in the bottom of the coolant passages affectng cooling effenciency in some cases.
The only way to correct is to mechanically remove this crud by removing the freeze plugs and rodding the rad.
A driveway method to flush the system after the conventional reverse flush is "boil" it. After you think your reverse flush is clean, try the boil approach. Block the rad air flow, let the car idle with a controlled boil over. That is, rig a set-up so that a small amount of pressure can build up, before it blows out, then add new water to replace expelled water. Add watter slow and at very small amounts to allow it to preheat before contacting block & heads. The mechanical agitation process of boiling water will dislodge huge chuncks of debris and gobs of sediment. Repeat several times.
I’ve done this effectively many times with old cooling systems without any undesirable consequences. It’s messy and ugly but almost as effective as formal cleaning process during an engine rebuild.