Rear Bumper Melted

[40daws]

!989 240sx,SOHC, Header,Catco High Flow Cat, and Rsr-Ex Mag . The car was running rich at first after putting all this on,so after new Efi Temp sensor,O2 sensor and cleaning out the IAC it ran fine. No more black smoke coming from the exhaust. Drove the car about 12 miles round trip after this and it ran great about two weeks ago. Drove the car this past weekend for about a 40 mile round trip.Again ran great,no problems. Get back to the shop and look at the rear bumper and the bumper has started to mushroom out and the paint is gone in an area about as large as a plum. WTF ? Even the impact foam behind the bumper got a littlescorched. I don't think the car is running rich anymore, I think this is just the remains of where it WAS running rich. But what do I about the rear bumper ? The Ex-Mag sit just below the rear tow hook,and just about it is where the plum/hot spot is ? I've searched,and other than SCC Dave Colemanms Project Silvia car where this happened and he put some sort of heat shield around the newly reapired bumper. I want to do that,but I also want that to be a solution. And then the question is where do I get the shielding ? Thanks Guys !