After doing some checking, it appears that the 116 models with the 617.95 engine had the temp switch in the thermostat cover, instead of in the housing itself.
What year is the 300D?

Another option might be to drill the existing bolt completely out. Then use a longer bolt with a nut on the back of it, instead of the bolt threading into the housing. (ie, the bolt will go all the way through the housing.) Looks to me that there would be adequate room for the nut.