Before changing anything, why not take it to the track to get a baseline? You may find that some simple tuning adjustments will get you closer to your goal before you start dipping into your tight budget. Make passes with and without Nitrous. That way you can get a better idea where you need to spend your money. Also, why do you think that 6,000 rpm is the limit on that motor? I had a .030+ 400 small block that regularly turned 6800 and lived for over 350 passes. And that was with some old 1.94 valve 461 casting heads with only minor port work done. That was without Nitrous, so you may be applying a harder load on the bottom end than I was.

Personally the very first change I would make would be the rear end gears. With your over drive set up you may even be able to go as deep as 4.56 and still be livable on the street. How tall are your rear tires? You can achieve some gear reduction using shorter tires as well. Have a set of taller tires for dailey use and switch to the shorter ones for the track. You should be able to get a 9-10" wide tire under that Chevelle with the right wheels and a little inside fenderwell massaging.

Keep us posted on your project.

Pat