MAke sure you have a wrench that is the right size, no play in it, then get under there, and use a floor jack to push up on the wrench to break them loose for you. penetrating oil doesn't hurt either.
Get some PB Blast and spray the s$!t out of it. Then wait about 5 min. If it's rusted then that should work. Don't use anything else but that, it works the best.
If it is still on the car how bout just unplug the the connectors and lower the hpipe and take them out outside of the car...much more room that way...
it is outside the car, i have my old stock h pipe out and im installing my new mac right now, its rusted, i dont have a wrench that is that big, so i had to use a cresent, and its just not working....
With the H pipe laying on the floor, use a breaker bar or a hollow pipe and put it over the end of the wrench and that will give you a lot more leverage. You wont break em so just give em hell, and soke them with rust penetrant. they will come off.
They are a mutha! I bent a 1/2" craftsman ratchet and broke a o2 sensor socket trying to get mine off. Luckily the aftermarket h-pipe I bought used had a set in it and the guy said they worked well so I took the chance and left them in. Dude didnt lie, they work fine...whooooo.