You know what? I checked on it, and it is a 76. The title says 77, but it has the body style of a 76. So yeah, how screwed am I? I edited this entire post to fix my dumb mistake. Thanks for pointing it out.
The engine on the 76 just looks cleaner, more simplified, more "vintage" looking, just a lot more suited to what I want to do.
I'm planning on painting the block John Deere green, swirl-polishing the valve cover and intake, using all chrome acorn nuts and bolts, and using polished and lacquered copper hard lines, and topping it off with some homemade open zoomie headers. It'll be unique, I can promise that.
I'm definitely not looking into fitting the OM615 into the W123 chassis, I'm going the other way around, fitting the OM616 into the W115. So, basically, I guess what I need to know is, what will it take to make the later engine fit in the 76? Will it bolt up to the auto trans? And will the 76 engine bolt up to the 83 trans?
By the way, since I didn't mention, I'm pretty decent with fabbing and welding skills. It would be nice if this were a bolt-in, but that's okay if it isn't, if I can't make something work, I have a good friend who has a Masters in engineering and access to a full machine shop.