You'll need to really soak the nuts well with a good penetrating oil. This will take a period of time, say over a few days unless the nuts have been off recently or they aren't very rusty. When you remove the nuts make sure they turn smoothly, if they start to bind, stop and respray and wait awhile. Install new copper or stainless nuts if possible and coat them with Never-Seez or other brand of anti-seize compound. Get all the bolts/nuts well started before completely tightening any of them. This will help in lining it all up. Hopefully the new pipe will line up perfectly but you may have to work it a bit to get the frame mounting bolt to go.