You must talk to a different bunch of people than me. In 15 years on this site I can only think of maybe about 6 times. (mine was one, but replacing it didn't fix the P0420)

 

Installing my new header and cat. The new header is touching the aluminum tube underneath it. Is this part of of the EGR? This isn't interfering with the fit, but I'm thinking I should flatten the header a bit and maybe wrap it with insulting tape?

 

That's the coolant pipe between the water pump and the thermostat housing.

It's been discussed before. If memory serves, the coolant pipe will rotate a tiny bit, but it has locking tabs that prevent it from rotating enough to clear the header. One might be able to find that thread with the search tool.

 

There is no EGR system.

 

Yeah, fit problems with aftermarket downpipes is one of the several reasons to preserve the original cat when possible.

The heat transferred to the coolant pipe will be no problem. I'd be worried about rattling, tho. Dunno if the force could cause a leak down the road.

 

A 2lb hammer would make some room on the header. I would not mess with the transfer tube.
Rattling noise would mean the 2 pieces are abrading each other. Coolant on the ground or CO fumes killing the driver; neither is good.

 

In fact you can see the locking tab in the first pic in post #22, on the left.

 

Well, I discovered that you can rotate the coolant tube, but on mine the mating surfaces were not perfectly flush after trying to rotate it. I could if tried to heat and bend the pipe, but that was too much work. I just flattened the header a bit. Also, I wrapped the header with fiberglass wrap.
Also replaced the CAT

Codes are now staying gone.