No real need to run the tank empty, just fill it up, drive until the needle gets to the bottom side of the 1/2 mark, refill then. You can do the same at the 1/4 mark too. Knowing those two refill capacities, should be enough to confidently drive the car, I would suspect. You will also notice it can take a while for the needle to drop from "past Full" at a refill, too. Notice all of these things as they happen. Then, only drive to 1/4 and not lower, as a general rule. Not to forget the massive amount of money it takes to buy 20+ gallons of gas at the current prices!
The main thing I see is your choice of carburetor. I suspect that something like an Edelbrock AVS2 would provide better results. The Brawler, from what I have seen, has lots of adjustability in it (air bleeds and all), but is not really designed for a "fuel economy" carburetor, or one which can get good fuel economy out of the box, as the Edelbrock/Carter carbs usually do. On the other hand, there are better choices from Holley, too. Just my observations, no more, no less.
What you should aim for is a carburetor which provides quick starts, efficient cruise, and good throttle response. No need for large throttle movements (which decrease vac advance in the process) to get things going or to change speed, just the opposite. Brisk acceleration (part throttle) "to speed", but then a steady cruising speed if possible. The OEM-spec Holleys I've had on several cars will do these things.
From the experiences of others in this forum and other forums, that "new sending unit" will probably NOT fix your tank level issue. Lots of work for little to no gain. NOR will it read any more accurately, by observation, either. Save your money and effort. DO ensure the sending unit is grounded to the fuel line, via the special fuel line clamp Chrysler used back there. Or replicate it with some hose clamps and a piece of wire with the insulation stripped from the ends. Just the observed nature of things. DO check the three sections of rubber fuel line between the tank and the fuel pump, to ensure they are not leaking/seeping, then evaporating, leaving no traces of leaks.
In order to get a good check on fuel economy, fill the tank and go for a 100 mile highway trip at 60mph or 70mph, on flat roads at a constant speed. Then refill the tank by the same method as before. That will give you a much better idea of how well things are working. Then go from there.
I have no issues with your intake manifold. A single point distributor is fine as long as the dwell is set correctly (which can get to be a problem with a high mileage distributor, from my experiences). I like NGK V-power spark plugs (and the later Iridium fine wire plugs, too), but I can also modify normal spark plugs to get similar performance out of them.
I see no issues with a 600cfm carb on a 318 engine, from my experiences with a 600 Holley on a Chevy 305. In theory, at WOT, the Holley vac secondaries only open enough to fill airflow needs, according to them. Highway cruise is not using them, anyway. Many 318s had 2bbls with 1.44" primary throttle bores, the Holley 600s have 1.56" throttle bores and efficient venturi clusters in them, so no real issues there, either. FWIW
Enjoy!
CBODY67
