Last heater core questions: I HOPE.

[In The Ten Ring]

So I am figuring out how it all goes back together and I think I just about have it. EXCEPT for this little guy and a rivet question.

I don't think there was any gasket caulk between the heater core and the opening it rests against but I have this rectangular gasket that came in the GMC Paul's kit. I can't quite get it to line up.....is it necessary?

(Disassembly pic, the current state is much cleaner with a lot less rust. )



Also, I bought 1/8 inch aluminum rivets with 1/2 inch of grip area, would those work for what rivets the heater box requires? I plan on using aluminum washers on the fiberglass side.