I haven't replaced the front pipe on an MB but I have on other cars (I used to work at Midas...hey, I didn't say I was proud of it ), and those studs and nuts can be a pain. Spray them liberally with PB Blaster or equivalent a few times, preferably beginning the day before you try to remove them. In extreme cases I've had to heat them with an acetylene (sp?) torch to get them to break. If you have access to an impact gun that will help. Hit it with short bursts of about half a second or a second each, and this will minimize the chances of snapping one of those studs. If you do break one, I'd look for one from a donor car before trusting studs from the hardware store. Good luck!

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1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K