Im thinking about buying the Percys Seal-4-Good reusable flange gaskets and I heard they were awesome. has anyone ever tried or had a problem with them?  

Are you referring to the multi-layer aluminum header gaskets they make? If so, that's what we used on our 427W stroker in our Cobra replica project and they sealed great!

HTH...
Mark  

The dead soft aluminum collector gaskets are good items.  I use them on the cars here.  They will eventually fail - I had one that only lasted about 1800 runs at the drag strip and failed by splitting past one of the bolt holes.  That's an extreme use case, for sure.  It was well worth the $15 or so I spent on them.  Over all, the dead soft aluminum gaskets have worked very well.  Certainly 10K times better than any paper gasket.  I imagine the same deal for the header flange gaskets - they will work very well, but will not last forever depending on circumstances.  But long enough that you'll get your money's worth.

I see they don't do 351Cs, so I thumb my nose at their technology  

For header flange gaskets, I use Felpro metal sandwich or Mr. Gasket ultra Seals of the same type.  I have had long life with no burnthrough out of those items.  But they come off usually every year or two because the headers come out for some reason - tranny work, etc.