You dont need to wrap your cats either, they get air when they are on the car dont they? As far as leaking from the header to mid pipe, most ball collector style should seal fine. Your leak would most likely come from the header flange to the head this is caused by poor quality headers or a warped flange. Stay away from headers from individual flange headers to help minimize this risk (MAC).
The o2s on the 5.0s just tell the computer how rich or lean the bank is running. On a 4.6 they have the same ones in roughly the same spot, but they have another set in the back that measures the exhaust after it has been run through the converters. This tells the computer that the cats are working properly. The 5.0s don't have these o2s.