Tight bolts don't mean its aligned. I've done this myself, it took two attempts before I finally got it right! Look real close at the intake manifold, especially the under side were manifold mates up with the head. It doesn't take much of a gap to cause the problem. The alignment rings get moved out of place, and won't let the intake seat on the head! If it fit before, no reason it won't go right back on!
