[Myka]

The 1 3/4" holes are for 1" bulkheads.

My educated guess would say the 2 1/2" holes are for 1 3/4" bulkheads, although I've never seen bulkheads of that size. Normally, the hole is 3/4" bigger than the bulkhead. Maybe someone squashed a 2" bulkhead in there??

I like thread fittings because you can remove or redo them. Slip fittings are more permanent. Often you have to hacksaw all your lines apart to get them off and replace several pieces if you have to replace a single piece when you use slip fittings. With the thread fittings you can unscrew (not as easy as it sounds though!), and replace the piece.