has only 6" wide rims, unless it's actually an SEL or a 380SL, in which case they might be 6 1/2" at the most (properly measured between the inside of the bead seats). But the critical issue is offset (30mm ET on your stock rims, probably -- it's stamped on the back) and the 500SEL will almost certainly have rims stamped either ET 25 or ET 21.5, either of which will work fine, moving the wheels outward on the hub only about 1/4 inch and thereby providing a little extra road feel, etc. If the car happens to have some of the aftermarket wheels with ET numbers more in the BMW style of the era -- as low as ET 16 on some AMG special models -- then you had better look closely at the fender clearance on the outside and expect a lot of road feel, a need for extra careful wheel balancing, and maybe some objectionable kickback on bad roads.

PS!! If you want the look of having the heads of the wheel bolts visible at the face of the wheel (the OE look), be sure to get the 500SEL's bolts when you get the wheels -- they are becoming a scarce commodity.