[Lowell W]

Did you leave a (some) bolt(s) loose where the header and sidepipe bolt together? Drop a bolt into the header?

Is the header touching something? I'm sure you checked this already, but if the header is touching anything, it would make an awful racket, although it wouldn't explain the difference in temperature.

Are you SURE you haven't mixed up a couple of wires? Maybe at the distributor?

A blown header gasket is a possibility, especially if you left a bolt loose. If you have checked all the above and haven't pulled the header off, now is probably the time to do that to check for a damaged gasket, but first use your stethoscope to listen around the header flange for a leak.