Fock... We have all been there. When I did mine, one seemed to pop loose, but actually it had slipped and started to eat up the head of the bolt. The whole process took me about 6 hours, but was worth it.
So to get to your question. It's been about 6 month since I did the cats, but you should be able to get 2 on each side from the top. One is visible, but the other is still accessible, but you have just fish around from the top, go where it should be and drop it one the bolt. Another option is to have someone underneath guide it to the right spot. I think I might have used a swivel right after the socket on that one. By the way if it starts to strip the head, the "Irwin" bolt extractors are a must... It bites into the messed up head and saves the day (you did soak them with penetrating spray, yes?). You can also access the bolts from underneath, but you really have to be patient. At first glance you would think no way, but if you use a box wrench with a slight bend on the closed end you can fish it over the bolt and drop it on. Here are some pics of the tools and if all else fails you can take it to a muffler shop, but you can really do it yourself.
