I'm not too sure on the gains (maybe 10-15hp), but I know what your talking about as far as threading the bolts. I just put another motor in my 86 mustang a few weeks ago and had problems with both headers. If I came to a set of bolts that wouldn't thread, I took the bolts out that I had already threaded and started at a different pipe. I had to use a prybar to flex some of the pipes, but I eventually got the bolts in.
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