Your kind of screwed if they welded your slip joints together. Was it the tabs at the joint they welded or the actual pipes. if the tabs you may have to cut them off or apart. Then the tabs have to be repaired or replaced (more welding). If you get them apart you can try sealing them with this.
https://www.amazon.com/Walker-35959-.../dp/B000CQDL94
If much of a leak though - I would not expect a miracle. If one or more header pipes are a loose fit you may need to get a tailpipe expander and expand it slightly until it fits in the collector tighter.