I have a 1980 450SLC, and believe that it's interior components will be the same as your 1977. I changed my stock wheel to one whose diameter is 14 inches. It was a universal six hole wheel (not Mercedes), and I used a Momo Hub Kit, #6002. The hub was hard to find, because it's a discontinued model. The wheel came from an Ebay auction, the the hub from AutoSeattle.com.
The change-over is pretty straight forward, and I am happy with the results. The only shortcoming, is that the smaller diameter wheel blocks my view of the top of the speedometer. Anyway, I always drive under the limit, so it doesn't matter.
I'm going to try to attach some pictures that may be helpful to you.
Good luck,