I just put a 34R in my S600 (had to fab a bracket because it's smaller).
Sam's Club can special order the Optima (sometimes you can get lucky and find one at Sears, but call before driving not all carry it!). Sam's got the battery in 3 days.
I also have a 34R in my 84 300SD - gotta love 800 cold cranking amps on a high compression diesel!!!
Just found the sams reciept - 97.07, after all the fees and tax - 115.66 total - their part # 106403 - hope that helps!