Wow, that's just nasty.
For a good compromise between fit and performance, reproduction ram air manifolds are the best choice. Get them at Ram Air Restorations. Good headers will out-perform them by a little, but in return you'll also get some fitment pain - pretty much regardless of whose you use. I'm running Doug's on my car and they fit "ok", but it has round port heads. I've no experience with their D-port headers. Another thing on the oil filter, make sure it has the right oil filter adapter on the engine. The right one for the GTO sort of angles the filter inwards toward the block slightly and gives a tiny bit more clearance.