1985 300D bumper fix

I have a 1985 300D on which the rear bumper has rotted. I am told that although the bumper mounts differ from the earlier 123 models, I can use the same bumper assembly without any retrofitting. Anyone have experience with this issue? If I'm not merely replacing the bumper, but also the supports, how involved is the installation? I'm asking because I have a source for the pre-1985 bumper w/ supports, but not the 1985 bumper alone (which would have the proper mounts). Thanks.