In the clubs I race in (Midwestern council of sports car clubs and the Madison sports car club ) His 560SEC with the 6.0 AMG motor ,suspension gears and exhaust would would add the following points :
Engine displacement change -1 point
Engine swap -non-original -3 points
gear ratio change -3 points
suspension change -springs 2-points
non stock exhaust 1- 2 points

This gets it up to at least 10 points and puts it in a modified class
running in AM modified in the high speed solo events and in AM in the slow speed autocross events .

In the high speed events The A class cars includes all AMG powered MERCEDES - C36 C43 E55 and all other AMG factory Mercedes--If they are stock they run AStock
you can see a full listing of all the cars in this class and all the other high speed classes and rules at:http://my.execpc.com/~mcscc/mcgcrv6.pdf

For the low speed autocross rules and class listings you can view them at http://www.madisonsportscarclub.com/ click on Autocross at left side and then on rules . Also the standings and results are posted there.

JUST A NOTE: IN the high speed open track events One of the Mustangs I ran against in FM would race in AM against your friends 560sec 6.0 AMG The other Mustang I ran against in FM would run in BM at the high speed event and my 400E would run in CM .

In SCCA SOLO2 racing my Mercedes 400E would run in the Street Modified class - This is where your friends car would also run I believe . You can find out by going to http://www.solo2.org/cgi-bin/ultimatebb.cgi/ubb/get_topic/f/5/t/000319 and asking in the forum for classing questions -The SCCA wants you to purchase their book on rules and classes rather than give the information out for FREE !!!!!