I'm not sure what you're asking but I think all you need is the length of the '75 automatic since you can measure the length of the '81 automatic yourself. I assume you've measured the '75 manual to be shorter (or longer) than the '81 automatic.

Will a 616.912 drop into a W115 as is or will the oil filter and other appendages have to be reconfigured like a 616.916?

Sixto
87 300D