My buddy has same setup on a short bed (shouldn't matter) he runs 275/60/ 15 on all 4 corners. He's got more offset on his wheels than stock too so you should be golden
Vortec gaskets. But if you plan to use these for performance purposes they are terrible. 454 vortec heads aren't like the small block vortec heads. The 454 ones are very low compression and poor flowing