Alright, so I'm replacing the downpipe in a '94 Lexus ES300. The bolts to the converter are seriously rusted so I was wondering if anyone knew if it was a nut and a bolt at the converter flange or if the converter is threaded to act as the nut? I thought of just cutting the bolts between the flanges but don't want to make it a huge headache on myself later trying to get the other half out from the backside if it's the converter that's threaded.

Typically those are not welded-on nuts and should spin free when loosened. A good yank with a socket on the bolt end will usually break it free enough to turn it loose...then just cut the bolt with a hacksaw or circular cutting tool. But if they are, you can just cut through the weld spots along the nut's edge with a Dremel or other rotary tool rather than trying to unthread a rusted bolt from it. Replace with stainless steel hardware if you can to prevent it from rusting again.