Cool 67.....take your time and you will have that great feeling of accomplishement when you get it done....there is nothing better than tackling it yourself.
Fitting that tail panel is a real bugger like Region Warrior said probably best to fit the quarters/wheel wells with the original tail panel still on as he suggested.
I am in the process of fitting a new tail panel on my 68 right now....I think I have had it on and off the car probably 15 times trying to get it to fit the best possible. Get a good spot weld cutter for the tailpanel....there are 1/2 million spot welds on that thing.
Are you an experienced mig welder? If not, a professional welder recommended a special mig wire to use when welding sheetmetal...its called "twenty gauge". It helped out my sheetmetal welding tremendously....I used to burn through quite a bit even on low heat settings...I have a cheap mig machine and wasnt very experienced and the "twenty gauge" was a savior for me.
If you do decide to buy a new Trunk pan.....dont get the one from Classic Industries.....the one I bought from them didnt fit very well....the grooves didnt line up with the original grooves on the original trunk.
Ricks Firstgen Camaro parts has a different trunk pan that is better.
http://www.firstgen.com/
Ricks actually carries some sheetmetal that is different than the other suppliers. Some of their floorpans, trunks and others stuff is made in the USA and considered show quality stamped sheetmetal. This is stuff that Classic Industries/Goodmark doesnt have.
Once you get going on the firebird you can cut into your 69 camaro....since you'll be an expert soon on firstgen Firebird/Camaro.