Holley Carb Jetting Help, Lousy Fuel Mileage

You might start with a simple lean/rich O2 indicator (MSD, Holley, others). I have bought for $25 on ebay. That will get you started and you can later screw a wideband O2 sensor into the same bung. Some give a little more than just an on-off signal. Double the cost if dual exhausts, or else run 1 sensor and assume both banks run the same. Wideband sensor kits are coming way down in price. Used to be $600, now ~$150. One advantage is you can install the sensor in the tailpipe, but I would rather weld in a bung than run such a long cable. I use a lean/rich in my 65 Newport to adjust my Holley Pro-jection. I can run a bit lean by turning the knobs a little more after the rich light goes out, or use it in feedback mode. What I see on the indicator and what I feel in how the engine runs are well correlated. I measured 18 mpg regularly when driving that 4000 lb car w/ 383 on the highway.