How difficult a DIY job is changing the intermediate muffler on a 90 420? Any special tools needed?

Sometimes teh bolts are rusted and they are difficult
to remove. I suggest first take a look at see how
bad is the rust. Try to soak it with rust penetrating
luricant overnight. You have a propane torch you can heat the surrounding of the bolts (not teh bolt directly), so that as the metal expands, it is easier to tremove the nut. Get your self a small metal pipe so that you can use it over the ratchet to give the extra torque. I had to off my bolts cut off with an acetyelene torch. Also, use valve grinding compound on the nuts you are taking off, this prevents slippage and rounding off the nut.
Good Luck