I think I found my vibration problem
friend of mine built this engine for me back 15 years ago, .
!!MAN!! You have more troubles
Seems to me you might have a "one off" balance problem.......that is it's not internally balanced, and it's not "weighted" the same as a typical factory 440 like a six pack. Odd combos of pistons and rods could cause this, and who knows? what he used for a crank?
So ?
1.....Are you SURE this is ONLY the engine, running, and not some EG exhaust pipe, part of the trans, etc, rubbing the floor, etc?
2....And it IS? only the engine? That is sitting there stopped?
3...."If" it's only the engine, have you run it with the belts unhooked?