Can anyone point out a post here or tell me what are wagon only parts, and more importantly where to get them?
Obvious ones are the gas tank(from what I read here), headliner, and tail lights.
If it matters I have a 74.
Thanks,
Russ

Rear suspension is somewhat different..... .........exhau st further back.......... ..rear electrical harness for sure.

rear springs - longer and handle more weight - not much more!

Shiny stuff? What's that?
right now I am not even sure what this thing is supposed to have. Between being a wagon and a 74, I am just about totally lost. 
Thanks,
Russ

Longer travel in rear end of wagon to be able to load vehicle with more weight. Rear shocks are different too. Fronts same as other.

russ ,easy fenders forward are the same doors back are different seats ,door panels and dash area are the same. glass windsheild same what parts do you need?

The rear shocks for sure are different length. I do not know if the shock rate is altered also.  Being cheap like I am I have altered a "regular" shock and it works well on my wagon (regardless, do at your own risk).

  I simply use spaces at the top and bottom to compensate for 1-1/2" (max you can get) of the 2" extra length that the wagon shock has.  This keeps the shock "centered" and frankly I have never had a situation where I have noticed them bottoming out.  The wagon rear shocks are harder to find (though available) and typically cost more. The picture shows the spacer on the bottom, but I have one on the top too. In fact, if you have a bunch of shock washers stacking them will accomplish the same thing.

Any more specific years/model of Firebird (and I assume Camaro too)?

If you gotta, you can cut the top off and weld on other, I did on my '80 when I couldn't find shocks for it. I've even rewelded bottoms but much riskier since sealed shock body there, you need a welder with patience to do it correctly.