I'm not sure if I understand the question correctly, but here it goes anyway:
1) for the pieces in the megaflow kit, although they tell you in the instructions not to glue together any of the pieces that are inside the overflow, I've found that water pressure (namely when you start the return pump) will force the pieces apart if they're just pressed into one another... so I decided to glue the whole return pipe together (and then to the bulkhead), and I also glued the very bottom piece of the drain pipe to the bulkhead;
2) regarding the underside of the bulkheads, you will need to glue any fittings (the hose barb adapter, if you're using hose) to the bulkheads, or you will have leaks (or a bigger disaster) sooner or later.
Hope this helps.
Nuno