The 'Full Tune Up' makes me suspicious... As does the fact it sat with old fuel in it...
I'd check a few things, but before that...
The 'Can Of Marbles On Acceleration' is DETONATION, and it is doing damage to your engine! DO NOT allow this to continue!
You are getting enough fuel and correct engine timing at idle, but when you load the vehicle, something is going WRONG...
Old fuel can plug up main jets in the carb, and that will limit fuel delivery and cause this issue.
So will low fuel pressure/fuel delivery to the carb. A fuel pressure test AT THE CARB, PAST THE FUEL FILTER is required...
Full Load means lots of manifold vacuum,
I would check for vacuum leaks under the carb and at the intake/head, look for open ports on the intake, or even a cracked intake.
I would check the vacuum advance to see if it's working properly.
I would DOUBLE CHECK ignition timing to make sure it's not way too high. Both at idle, and with the RPM ran up to about 1,200 RPM, vacuum disconnected. See what the mechanical advance is doing in the distributor when you gain RPM.
What is your tire size and gear ratio? Some people with 2.73:1 gears slap on tall tires, and that will over stress the engine and cause detonation... With tall tires the drive line is putting too much stress on the engine...
Does it do the rattle in all gears, or just the top gear? Does it get worse the higher gear you are in?
Does it do this when you are highway cruising, then open the throttle a little? Go away when you downshift?
New exhaust? Catalytic Converter? New or the old one? Cat converters plug up and cause issues also.
These questions will allow us to diagnose the issues a little better...
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