i agree, it could be possible, however the seller of the car did reference an known builder who went through the engine and I am making the assumption that they wouldn’t have put the wrong balancer on it.
I agree, it could be a witch hunt and I really don’t want to go through the whole drivetrain but in the end may have too.
all of this already makes me want to scrap the headers in favour of manifolds as it will make removing starters and trans inspection covers to remove the trans and try converters etc….. also in case I have to remove a motor to disassemble and have balanced etc…..
but based on it the being a pulsating vibration (in and out at the same repeatable rhythm) it is pushing me towards transmission. If it were motor wouldn’t the vibration be constant, and all the time through the whole rev range??
I have dealt with an unbalanced Mopar setup (Cast crank motor that had an incorrect flexplate on it) and would shake the Glass out the car at idle. I am thinking this may not be your problem.... It does appear as though it is something past the Engine. You can disconnect the Driveshaft and spin the Rear axle assembly to see if there is a constant eb and flow. I have also seen a Driveshaft too long creating a Vibration issue but it's been too long ago to remember the details. I would try to eliminate the easiest and least costly to begin with.
JW
oh yah @oldkimmer the exhaust isn’t touching anything either.