I would use a late model LA block or Magnum for the next build to be able to use a roller cam. The LA blocks from 89 to 92 I believe had provisions for roller cams and all Magnum motors had them. A roller would give you a nicer idle with a bit more aggressive profile than a comparable flat tappet. Magnums have the added benefit of coming with a superior head.

If you don't plan on going much over 300HP I would keep the factory manifolds for the ease of working on it, ground clearance and quietness. Much over 300HP though and you start to leave a lot on the table without them.