Buildcraft's iron pipe and Quark's item pipe have the same recipe. This makes the Quark item pipe unobtainable in any pack with Buildcraft, which isn't exactly an uncommon mod.

I recommend replacing the vanilla glass with Quark's framed glass. Since it would require slightly more iron, I'd also increase the output from 6 to 8.

You can also use the mod NoMoreRecipeConflict.