MAXIMUM Header Gasket Thickness?

you have to really torque down on coppers to get a good seal on 'em.

'ol trick for paper style gaskets is to submerge/soak the gaskets in water for a good while. (should help if you double up). this will cause them to swell up a little (aka conform better). Install them, and let 'em dry out for a day and they should seal up nice.