2.8 engines date back to the sixties
I would be guessing this will be an old M110 2.8L with 722.3 transmission form either a W123 280e or W126 280SE
here's the M110 wiki : https://en.wikipedia.org/wiki/Mercedes-Benz_M110_engine
Here you see old Kent talking about the m130 engine : https://www.youtube.com/watch?v=DE6acpIhpkM