I had Ford Racing, and they started rusting out on me, even though they weren't supposed to. I called Ford Racing, they said "huh, those shouldn't rust." I asked if they could replace them, they said "no, no warranty on them." BBK ones will turn blue on you, or orange perhaps. Try to avoid the plain painted ones as they will turn to junk in just a matter of months as well.
My BBKs blued within a few weeks and when I took them off they had some surface rust in some spots. Never had any problems with them as far as fit went. I changed to the FRPPs because I went with gt 40 P heads and I got a good deal on some ceramic FRPP headers. Nice stuff. Personally I would go with some FRPP ceramics if you can swing the extra cost. Hunt ebay maybe.
I have the chrome BBKs and they work pretty good, no rust and I have had them off and on the car three time easy as the first time. Only thing I would do different is get the ceramic ones. Good luck
In response to your original question, on a mostly stock car you'll probably not see any HP gain at all with the addition of those headers.
The stock headers, while they look restrictive, actually don't pose a restriction to peak flow/power until the motor gets up around 280-300 hp.
I lost 3HP going from stock headers to FRPP shorties on a mild combo. There are many other examples of this out on the sites. Long tubes would be a better choice - might not help peak much, but would probably support a nice gain in low end torque compared to the stock headers or FRPP shorties.
Be careful of claims to the contrary -- be sure dyno numbers before and after were only for a change of headers. Most of the claims of 10-15-20HP are for entire exhaust system changes.
If you want to spend money on something that's gonna make a difference on a stock car -- have tmoss port your stock lower intake. That'll make a much bigger difference than a set of shorties for about half the price.