air space

I got a 76 Firebird. I'm trying to put 2 15s in the trunk with a (not built yet) fiberglass box. I'm looking at put one sub in normal and the other reverse mounted. How would this effect the air space? Each one sub calls for 1.5-1.7, so I need 3-3.4.

I'll problay only put one of them in  because I only have 1.2 cu.ft

I've yet to buy the subs, and I do have more room. Just not to much, my trunk is more narrow/shallow. I do got enuff room to make a single 15 work correctly.

The reason I wanted two 15s was because I got a single 10 in there now.......and its not cutting it at all. I'm really thinking about making the rear seat fold down and put two 12s or 15s instead.

Oh ya, on top of that its got a 455, full length headers going to cherry bombs to turndowns. So my exhaust stop right under the front seats  (hint..hint...its a little loud) I'm going to put some kind of sound deaden kit in there to help.

The answer to your question is guidance. This vehicle is a little more of a challenge than others due to the weird shapr of the trunk. Unless you are fully prepared to do and tackle a custom sub box for a good sub, do not expect to get above average output from any sub. Good bass response is all about the box. Two 10's or even 12's can fit, expect the box though to make or break the output of them. Not happy with what you have now? Look to a new box system first, then choose a sub for it.