Lots of good advise so far for how to improve things, everyone can have a different take on things. This is what I see. Assuming starting line weight @ 3550#, it would mean that your making approx. 310hp at the crank, or 250 RWHP. In saying that, it would mean your ET should be in the 13.35 to 13.60 range to correlate with that weight and MPH. Taking a shot at doing the math, it would indicate your crossing the line at approx. 4,400 RPM (4,250 w/3% conv. slip) Your overall first gear ratio is 8.28, and that's hurting your 60ft big time. The 383 ci is taxing that intake, and your under carb'd as it is. Do you have, or do you know your fuel pressure crossing the line? A change to a 4.10 gear with those tires will help out big time in the ET dept., but will not make much change in the MPH dept. as your HP will not have changed. As noted from others, this should be a mid to high 12 sec. car. Cylinder head also may be getting into choke due to design, has nothing to do with CFM/CC's.