Cylinders 5 and 7 Fire consecutively and as such rob fuel from each other too much if things are on the lean side especially as rpm / firing rates go up.
Shapes and area changes in intake manifold runners can make a good amount of the fuel drop out of the air stream and just coat the port walls before it makes it into the chamber.
This all adds up to rod pounding detonation as you have experienced.
When you get things back together I would jet up that driver's side rear of the Carb atleast 3 jet sizes or whatever jet increases are needed until you can get the plugs in 5 and 7 to look a tad fat, also a Carb sheer spacer like the Wilson one might help if the big combined back flow pulses from 5 and 7 are playing havoc with cylinder 7.
I would also consider getting probe into 7s Exh tube to confirm that cylinders running temp less you want to be going through this whole rebuild again.
Also please post up close up pictures of all of your plugs noting which cylinders they came from if you would?
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And he was not talking about 1/8 or 1/4 mile ETs!
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Two dry rotted 14 x 10 Micky Thompson slicks.
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Two pairs of brown leather flip flops, never seen more then 2 mph.
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