Reply per John's question: The 250 is internally balanced so any flywheel for that inline family, or SBC V8 up to 1985, that bolts on should work. The exception is the flywheel from the 400 cube small block Chevy as that engine was externally balanced. You can use either the 12 3/4 inch 153 tooth wheel or the 14 inch 168 tooth wheel no problems. The flywheel size of course depends on the starter and clutch size you are using.
Assuming you have a correct flywheel for an internally balanced 6 or SBC V8 I can only think of a couple of reasons that relate to it. First is someone had that flywheel balanced with a crank attached to it and they did all the weight changes on the flywheel. Dumb yes, but it happens. Second is the pressure plate is way out.
Generally it is best to have the flywheel and p- plate balanced as an assembly minus the clutch disc. Once balanced the machinist should have marked the 2 pieces so they can be reassembled in the car in the same relationship they were balanced in.
If your truck has an automatic try unbolting the convertor from the flex plate and indexing the studs around to the next holes. You only have to do this twice to cover all 3 locations that can make it better or worse.
Putting the engine aside for a minute - is the vibration at 3500+ only while driving or 3500+ at any time driving or parked?
Toss some more info to us before you go ripping into the engine. Driveline problems need to be ruled out.
Mike G
