Had a 1973 Dodge B-100 van with a slant 6.
Swapped the engine out one winter. (outside) I don't remember why. It had a sometime vibration before the swap. Not the reason for the swap. Still had it after the swap. Annoying vibration, but not horrible, any not all the time.
Years down the road. Spun a bearing, or maybe more than one. Replaced some bearings out in the yard. Ran it more miles.
More time passed. While returning from NY to Michigan from leave after leaving a Thruway stop while getting back up to speed the oil pressure light came on. I was able to make it to a friends house to leave the van. Rented a car to return to Michigan, and looked for an engine.
Found a Plymouth Duster with a frame that was giving up as a donor vehicle. Had grand visions of driving it back to NY to swap out the engine. That plan fell through as the rust out proved to be to much for the front suspension to keep the tires from rubbing in places where they should not.
Borrowed a Toyota Landrover, and towed the van back to Michigan. Did the engine swap on base at the auto hobby shop. No more vibration. Difference was that this time I swapped the car engine flywheel, clutch disk, and pressure plate along with the engine. I was going to keep the larger clutch and pressure plate that was in the van, but just did the straight swap even though it was smaller.