Carb question stock 340
Having owned a '68 340 for 54 years, I found that Carter/Edelbrock carbs were easier to tune than Holley and similar cars. Metering rods are far easier change that power valves and channel restrictors, and older Carter's have 3 stage metering rods. I ran Holley's but was not impressed with them over the AVS. The Holley carbs always got lower MPG by a lot than the Carter's. I was able to tune a 340 to get 24 MPG during the late '70's energy crisis and a Holley would never perform that well.
And fuel economy is not just paying less for gas. Fuel that is wasted during combustion ends up either washing the cylinders causing wear or ends up in the oil, causing wear and deposits. This is why a modern FI engine lasts longer than an older engine with a carb. They simply have better fuel management.
By far the best compromise is a Thermoquad which has both the air flow for power and small venturis for fuel atomization. Best of both worlds. But it does require more care in assembly but no difference in tuning over an AVS. There are pleanty around but you do need an adaptor for a square bore manifold or an Edelbrock RPM with the spread bore carb mount. This is what I am doing for my restored 340 which hopefully will be fired next month.