Everything is leading back to the crank.
Looks like they took the stock 1970 318 engine, stock pistons and stock rods, and through a 4" stroker crankshaft into it. That would not have matched up to the proper balance for that engine build.
Will it run > Yes, will it be right > No.
Hence the low end vibration.
Another test, mount a half full clear water bottle to the intake/valve cover area (duct tape is your friend).
Then run at an idle and watch the water vibration, then rev it up to 2000 rpm and watch the water vibration again.
Out of balance engine the water will be jumping through the rpm range change.
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