What is "Parping" exactly?
I think 260 is almost in the red.....but.........if you had coolant flowing vice blowing out coolant reservoir....and if only for short time.....
Guess I would check compression on all eight cylinders and do the secondary check with plenum in place.
Keep an eye out for oil in coolant and coolant in oil or coolant leaks anywhere. If this "Parping" was a result and directly related to your over heating issues.....then something obviously changed during that heat cycle....somewhere.
Checking Function of Secondaries with Plenum In Place
A sound that parps, as of a horn or trumpet
There is another definition of that sound involving a P-38 pistol.....
Now if the engine no longer makes the "Parping" sound and
only when it was very hot.....I think the "Parping" sound was coolant boiling over popping out the pressure relief of the radiator cap on top of the coolant reservoir in front of the passenger side. Or......cavitation of the water pump as steam pressure built up in the coolant system where you exceeded the boiling point of the coolant.
But......I see you say you hear "Parping" above 3500 rpm SINCE the overheating issue.
Or.......you have head gasket leak and excessive air pumping into the coolant and out the pressure relief of the radiator cap on the coolant reservoir. Check coolant overflow container during idling after the engine has reached operational temperature to see if there is a lot of bubbling into the coolant overflow container located under the passenger side headlight.