Im using a 94 T4 Transporter / camper and putting in a 98 Golf AHU TDI engine.
They are pretty much as efficient as it gets if you look at the bottom of the page on the link below you can see how I have worked out the MPG. Its fascinating !
On the aircraft engine we are a 2 stroke diesel and running direct drive. We have a pretty sorted balance system and do pretty well vibrations wise all things considered. Mass is the hard thing though as you say getting it down but keeping up the strength. All good fun !