IMHO,
you for straight fuel economy
you need to get the cruise rpm down to something like 65=1800, then reengineer the advance for whatever she needs, then tune the carb; any carb.
But, to run 1800, with a factory 340 cam and headers is counter-productive for two reasons;
1) with the very late Ica, the lower rpm that you run it, the more time becomes available for the pistons to push just inducted A/F charge, back up into the intake, and
2) with 44* of overlap, the headers are gonna "suck", some of that right across the chamber and out into the pipes
3) both of these actions are counter-productive to fuel economy
So
if your 340 still has a factory spec cam, then get rid of it.
>If she also has headers and you want to keep them, then you will need a cam with less overlap, a longer power-stroke, and if you need more pressure then an earlier closing intake. To get some power back, your cam needs to have a higher intensity. That is to say, faster opening and closing rates.
> While the 318 cam is a good candidate, I feel that there are many better options.
>But if you are running log-manifolds, which kill overlap, then you can run a lil more overlap, as may be necessary, to satisfy the other requirements.
>After the engine has been set up for a higher efficiency, THEN, you can consider a lower cruise gear. How low will depend on the installed cam.
For selecting the minimum cruise rpm, here's what I do;
Put a vacuum gauge on the intake. Warm up your engine. In Neutral, rev her up slowly, bit by bit for as long as the vacuum increases. Once you get to about 1700 rpm hold it there for a few seconds until the gauge stabilizes. I put it up on the Fast-idle cam, or wedge a shim in the curb-idle screw.
Next, without regard to the actual numbers, pull some timing in, bit by bit, until more timing does NOT produce any more rpm. Write the vacuum number down, together with the advance number. Then increase to 1800rpm and repeat. Then 1900, then 2000.
At 2000, fine tune the rpm by playing with the mixture screws, to achieve the highest rpm. Continue until the vacuum no longer increases, or say 2800 with a really big cam..
At whatever rpm produces the highest vacuum, this is the lowest rpm that your engine first becomes efficient. If yur going for fuel economy, then there is NO POINT in trying to cruise at any lower rpm. Higher rpm is ok but, economy may suffer and usually does.
Sometime after 1800 rpm, depending on the cam, your vacuum will plateau for a few hundred rpm. You can cruise anywhere on that plateau and expect similar fuel economy. declining a hair with increasing rpm. Eventually, the vacuum will decrease, and you wanna stay away from that.
BTW
you can do this experiment, with your current engine, exactly as it is. For me, it has worked with every cam that I have tuned, up to the biggest which was the Mopar 292/108. My current cam, 276/286/110 (230/237@050) cam cruises best economy at over 2200, so mine is set up for 65=2240. But it's still lousy, cuz of the late closing Ica, which, IIRC is around 66*. It is what it is.
BTW-2
The 340 cam already has a modest 44* of overlap, which with log-manifolds, one would think it would be a candidate for good fuel economy. And the cylinder pressure of the early ones was great. But, the late-closing intake valve (~66*, just like mine), killed it.
As I remember it, the 340 cars that I have had, ALL of which had 4-speeds and usually 3.55s, all used to get best economy at over the speed limit, which in those days was 70mph. Except the one that I installed a 318 cam into ........ and sold the 340 top end of, in favor of the 318 top end. Hyup, that was a torque monster!