I had two 350-2bbl Firebirds. 1968 and a 1969. Both were automatics with tall rear axle ratios. The '68 received headers and duals to replace the single exhaust. Mileage improved from 17 to 21 on trips. It did not seem to make much more power at all however. I installed a 69 GTO Quadrajet and iron intake. At 3000rpm power just flattened out just like before. It flat spotted a cam lobe so I replaced it with a Pontiac 067 cam and a new timing chain. There was a huge improvement in power. I finally saw the power I was looking for. The '69 already had a bad cam and bent pushrods so I replaced it with a Crower RV cam and true duals on stock manifolds. It stayed 2bbl for awhile; it also exceeded 20mpg on trips. Eventually I found a cast iron intake and a Quadrajet for it; power was as good as the '68. Don't install a 4bbl without improving the cam.
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