How many of these headers exist in this country? IIRC you had these made as a template for more to come, so they're not that widely available. I would take that into consideration. Also, if you're selling the car in a year or two, sell the headers at that time.
I hope you realize that they make heat insulation for these exact same reasons. You can either get header wrap to keep them from radiating too much heat, or protect the parts that are getting melted with insulation:
When I had the headers put in the Regal, Bob had to bash in the firewall quite a lot with an air hammer and as a result, it started heating up the downpipe valley and melting the carpet. Bob shut the car down before anything happened, and I picked up some of this stuff:
It does an excellent job of insulating heat. Granted, he used two layers because the sheet I bought was very large, but it did the job nonetheless, and they do have thicker materials.
Look through their products. They have solutions from pipe wrap to kevlar insulating sheets that you can buy in different sizes. Since you paid nothing for those headers, it shouldn't be too hard to drop $60 in some insulation material to enjoy them before you sell the car, then take them off the car, put the stock manifolds back on, and sell the headers anyway.
2005 Pontiac Bonneville GXP - For Sale ----- 1995 Buick Regal GS(X) - L67 madness
2000 Buick Regal GSE - Stock...for now
----- 1987 Jaguar XJ-* V12 - 12>8