Conclusion
The Holley Street Avenger carburetors are designed to be calibrated correctly straight out of the box if they're used for their intended application (and there's a chart on the Holley website to help determine this), though admittedly the 670cfm carburetor is slightly large for a 305ci engine and we'd probably have been better off starting with a 570, but then we probably wouldn't have been able to improve on the stock settings! Given the engine's stock state of tune and its stock cam, it wasn't going to respond to tuning much beyond what we did, but we did pick up 4 lb-ft of torque and almost three horsepower by installing smaller secondary jets. The motor also ran slightly leaner above 3,500 rpm, though admittedly its cruising speed in third gear is around 2,300 rpm, so it's a somewhat moot point on this vehicle.

Out on the road after the test however, the pickup was more responsive, with better acceleration, though I haven't driven it far enough yet to report any change in gas mileage. While the FAST meter is of more use on a performance engine, you can see how it is useful for tuning fuel delivery. 14 parts of air to fuel is where you'd expect to see a carbureted engine in the lower rpm range, so we were in the right area to start with, and managed to improve the ratio at higher revs while increasing both peak horsepower and torque. Sounds like a successful morning's work to us!