I've seen the double head gasket thing done, and seen it on shows and read about it in magazines. Dunno about longevity, but they were able to make quite a few dyno pulls that way.
gt40 and gt40p heads are very diffirent.
351w heads have offset exhaust bolts, just like 302 heads. gt40p heads are side by side.
so yeah, you will probably need diffirent headers.