I put in 3 fittings - one 1+1/2" from the head
, one 1+1/2" that immediately Ts to pump-out+macerator, one 5/8" (or 3/4" can't remember) for vent. If orientation is flat/horizontal there's already a threaded fitting in the middle you can use for the vent. Not sure that fittings size and type though.
Making sure the discharge is at the bottom can be tricky, but they make a specific fitting that helps with that. In my case it's in a vertical space, with discharge at the bottom, so I didn't use the special fitting. It doesn't fully empty when pumped out, but that hasn't really been a problem for us considering this was a step up from an 8 gallon tank.
Yep, it's vented. While vents are not as effective for bladders, you've still really got to have one. It also will provide pressure relief just in case...
I didn't fill it first, I just sort of layed it out and figured out how it would all go. Putting in the fittings is a bit tricky and took four hands. Once you get the first one figured out, the rest go pretty quick. Note that if you fill it first you have to find a way to get the water
back out as it'll need to be empty+dry to put in the fittings (and you probably want to do it someplace where you've got room to work anyway).
I lined the hull
with some rubber mat from the hardware
store just to provide some cushion against any pointy bits from fiberglass