Pics might help someone on here give you better help and see a little better what is going on. Sorry, I haven't worked on one or with those parts.
My boss bought a dynacorn 69 camaro shell, and other new parts, fenders, hood ect. I didn't work on it and see exactly what they did, but do know they had some issues with fit, and it took some beating around and adding on to get good gaps. I think they said the right fender was really bad. Seems to be the norm with many of the aftermarket parts, need some modification, beating around and bodywork to get fitting correctly, but unfortunatly it is often about the only choice you have to work with today. Also the older cars didn't exactly fit perfect from the factory.
Did you try any sites dedicated to the firebird, that will likely get more view from others working on the same car, and more likely to have one find your thread who has already dealt with those parts? Here is just one site dedicated to the firebird, with a forum, and also read through the faq's and the thread I'll link to.
First off make sure your subframe or anything isn't bent (did your old parts fit fine?) and that everything is sitting level like those faq's say.
I'd hate to give you wrong advice and have you ruin anything, but some possible ideas, so take it fpr what its worth, not having previously worked on that car or with those parts.
If your hood edge bows in the middle but is flush in the front and back, you might possibly able to place a block of wood under the areas where it is flush, and persuade the edge flush to the fenders in the middle by using a decent sized block of wood and hitting with a hammer along the edge or maybe even putting some of your weight and pressure on it will be enough.
You say the fender gap to hood is tight.Its a lot tougher to take off the edge to get the right with gap then it would be to add on to the edge.
Is the door to quarter gap flush and is it even with the rocker? It might be possible to work the fender edge flush with the door if you set your hood to fender gap. But the top and specially the body line could be difficult. Is there a fender bolt iinside the door when you open it, that should help hold that area in flush as well, if you can get it pushed in far enough.
Sometimes when persuading something like a fender edge or a door edge in, I'll even sit my butt on the floor and use my foot to put some pressure on it. But if the top of the fender sticks out from the door when you set the right width gap at the hood, maybe you could get a wider fender to hood gap if you bolt down the fender and set it flush at the door, and use a block of wood and hammer along top edge of the fender, and hit out away from the hood to get a wider gap between the fender and hood.