Replacing Cats in 99 4.8

I purchased 2 Magnaflow Hi-Flow catalytic converters. Can anyone tell me what the easiest way to get the old ones off is? I just plan on dropping the exhaust post-manifold and working on it in the clear, probably cutting with my 4" grinder. Also, are the magnaflow universal cats weld on or can I also clamp them on ? There is abour 3" pipe off each end of the cat. Thanks.