The crossfire has the 3.5l v6 merc engine and gearbox from the older SLK ( As its just a re bodied SLK - including interior.)
This is what is in the std 300
The SRT crossfire was a very rare beast indeed but I dont think it was a V8 ( Not sure) but a supercharged V6
Yes I was speaking rubbish, srt crossfire has supercharged 3.2 v6 same as the amg mercs, like my old slk 32. So ignore my ramblings