Gas tank swap

[stingray72]

Ok, this has probably been covered but most of the threads I've found are for the filler locations and quite frankly I haven't gotten there yet.

Anyways.. I started cuttong off the spare tire crossmember last night and tried to test fit my tank (blazer tank from ****************). I noticed there are a few things that I don't know what they are? I've attached a stick drawing looking towards the front of the truck as if I were standing over the tank.

I've got a pretty good grasp on the filler neck and the sending unit hole but there is a couple of other openings I don't know what they're for. I assume maybe a vent or something

There's about a 1" neck next to teh filler neck.
and there are (2) 1/4" or so necks at the corner of the tank (top left and top right)

What are these for? (and forgive my crude drawing)