Hope you have some time available. I am in the process of grafting in a Chevy SBC into my '68 W108 with a GM 700R4 transmission.
In theory, it is rather straight forward. The challenges lie in the following:
1. In order to drop the engine as low as possible and as far back as possible, and to have the Chevy engine mount points be over the Benz engine cross member, the oil sump and oil pump are in direct conflict with the stock Benz steering mechanism (drag links and steering arms). Furthermore, the rear left exhaust port and spark plug are trying to use the same space as the Benz steering box.
1. slice open the crossmember, reinforce it, and modify the oil pan for room and still retain a good oil capacity.
2. Replace stock Benz steering with rack and pinion unit. This cures many issues and poses a new one - possible bump steer due to steering rack location (no clear solution as of today).
Now there is a fellow in NC, who took his 108 to Fatman Fab. and had them install one of their Independent Front End suspension system. It looks pretty sweet, but don't know what he dropped on it.
The next issue is fitting the transmission into the car. The GM transmissions are not very petit and depending on your selection, you may have to do some cutting and welding to make it fit. Then there is the custom drive shaft and exhaust system.
As you are playing with a W116, some of these issues (steering system) may not be a problem at all. Feel free to PM me and I'll fill you in more on my findings.
BTW - if you don't have the 454 yet, you may want to upgrade to the 502 cu.in. for some real fun and brutal power....... That engine will require the GM 4L80E automatic transmission or a Richmond manual trans to hold its monsterous torque. - and buy some stock in the tire company you will be using.......
Good Luck and have fun.