First of all, I am assuming that we are talking about a 3200 Lbs. Firebird.
IMHO, it sounds like you have everything you need already, and you should be well into the 13's without any help other than a cam, gears and headers. (The 3.55 gears you are looking for will not fit the 2.93 differential carrier, though any gears you choose will fit into the rear end housing. You will have to buy a series 3 differential carrier, which you already plan to do.)
However, I'd strongly recommend that if you're going to give it a shot of nitrous, you re-think your position of not doing something with the botom end. Pontiac's stock cast rods are good, but they should be heat-treated, re-sized and held together with ARP rod bolts to hold up under the amount of horsepower you want. (It's the rod bolts that are usually the culprit!)
You are right about the cam - more than 230 at 50 will rob torque from the low end, but you can get it back with either some cam advance and/or by staying with the stock cast manifold. Repeated testing has shown that the stock cast manifold with a CR prepped Q-Jet out-performs all others up to about 5,500 - 6,000 RPM. (I'll try to post the link.) Use Rhoads lifters and you'll have a silky-smooth idle, even with a cam like that. No need for a roller cam if you're not looking any lower than 13.
Heads and headers will make the difference. The 4X heads are good heads to start with. They don't take a whole lot of work to get them where you'll need them, unless they are the small-valve, press-in stud variety. (These would be marked -1H or -3H somewhere on a boss above the left two spark plug holes.) Also, the 4X heads have a smaller exhaust valve than the earlier 400 heads, and require a little more work to flow better. Banshee is the head flow expert. You really would do well to abide by his suggestions about what heads to use and what to have done to them. Of course, if you are going to pay for as much work as you're proposing, you may as well cut to the chase and buy a set of 87cc E-Heads. E-Heads will also let you run a full point higher compression ratio without detonation.
For the exhaust, you can spend a lot of money on Hookers, or a lot less money on Hedmans. The Hedmans will perform as well, and fit better. Get an "X" pipe in there, and a little more of that bottom end will come back.
Pump gas, a smooth idle and over 450 HP is easy with a 400, even without the shot. Getting that to the road efficiently is another matter.
Bjorn's right ... I should have sent this as a private message!
Thanks, Bjorn!
