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So my MX is pretty much gutted and tire well is cut out for my 350z exaust system . Im trying to reduce the road noise/ exaust note considering most for my interior is gone. So I have some ideas.... I already started to wrap the hatch/rear area with roof flashing. Which I am already really supprised with alot of rattle/squeaks are gone. But as for the "dead space" between the body and quarter panels what should I fill it with? I was thinking a mixture of home fiberglass insulation and polyfill which on paper it looks like it would work but will it? I will post pics of my project and write a review on how it works out. Any help input would be great!!! thanks guys!

Here is what it looked like when I first started:

You'll want to fill the edges of your metal plate well.. I've been looking into sound insulation, too, and what I've found is that having an open space between walls is good for insulating sound transfer, so if you put something on the inside of the external body panels (I did carpet cushioning because it was free), and seal the interior support "walls", so they are walls, themselves, and leave the space in between, that will kill almost all the noise. Check out SuperK's post on what he did, too.
I also sealed the gaps where my doors, and hatch meet the body. It's in my worklog. When I'm done, I'm going to carpet my hatch area, since I took all the panels out behind the seat. basially cover any metal with carpet.
Also, I'm going to undercoat my wheel wells.

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I ended up using two 30lb packs of cellulose at 9 dollars/bag

Not fiberglass, it isn't really meant for sound deadening and it absorbs water big time. Use Rocksol insulation. It is meant for sound deadening and it dosen't absorb water. Bag it of course. If you have the money find someone who does spray foam but it's a little pricey.

thanks for the ideas I will be using the cellulose instead....

Should take off the rear armrest panels... you'll find a surprising ammount of space behind them to fill up.

Ah. I skimmed over your paragraph and looked more at the picture. Apologies.

^ thats what I intend to fill with the cellulose

Just to clarify, you can fill the panels starting from where the seatbelt is, on back. Take the extra time and be thorough, it's worth it. There are a lot of nooks and crannies that you have to strain to stuff, and it takes some exploring to find them, but I think it's best it takes a while.
I used almost two bags. That'll give you kind of a idea of how much you can plan on using... if you use less than one bag, there's a lot that you missed!

I also stuffed from the inner frame to the interior body panels. Now the panels don't squeak a bit. Very nice, I like, why not?

the cellulose will absorb water?

how do you sound proof the doors without stopping the windows?
also it seems water gets in the doors and needs to drain out the bottom

The doors are an especially good area to do my method. You can put the flashing stuff (Dynamat) on the inside of the exterior wall, and seal up the interior support wall. It will make your speakers alot clearer, too.

Hey guys those expanding foam in a can work wonders, I frequently use it to seal off door speakers, makes a great insulator in hard to access places where you cant quite get another filler.