If fuel mileage is important enough to you to mention it, then perhaps you should consider your build as a big-bore 318, sorta like the 383 is a big-bore 340.
But since you are starting from the block up, you can chose a modest cam that will have some top-end potential, without killing mileage.
What that means to me; is a fast-ramp,solid-lifter, tight-lash cam that slams the door on intake charge before it has a chance to mess up the plenum..... because to get the big mileage numbers, you are gonna have to bring the rpm down.
And that means modest rear gears.
To get the low-speed performance back with an automatic, will require a higher-stall TC... but that works counter to the cam, so it will need to be a loc-up model. And that means you need a lock-up trans..... you see where this is headed?
But now, with 2.94s in the back,60mph is gonna be ~5900 in first gear.So as you said; not a speed demon.
Since this is gonna be a one-gear to 60 mph car, you might as well optimize it that way, and just install an A999 with a 2.74 low gear in it, and run 2.76s for 60=6200@10% slip... at the top of first gear. It doesn't have to be fast up there, it just has to go there before you shift.... just to make it fun.
Ok then, with 2.76s, 65= 2230 with 27" tires, in loc-up, so that is gonna have to be satisfied by the cam. Are you still with me?
If you don't already have 2.76s and an A999, and don't like my plan, then one of us is in trouble,lol.
But here's the deal;a tired 318 can make 22mpg with this combo, running 135 psi cylinder pressure, in a 3600 pound barn door Swinger.
Surely,your 340 can best that, because yur not gonna build a 135psi slug.
To satisfy a decent cruise vacuum at 65=2230 ( I have that), you cannot run a 276/286/110 cam ( I have that). But you can run a 223@.050 Solid lifter cam, which will make tremendous torque in the rpms your engine will spend most of it's life. Since this is a one gear to 60 combo, you also don't have to stick with a 112, or a 110 or even a 108LSA. But to take a 223*/5000rpm cam to 6200 is gonna take some decent heads and a tricky cam. (I took mine to 7000/7200 all the time, so 6200 is nothing,lol.)
So now you just go find that cam, pick your small closed-chamber alloy heads, and set your cylinder pressure to the highest your local gas will support without detonation, at your elevation.. Badaboom
And then you carefully assemble it, and don't forget the oiling mods,
And then pick your bolt-ons,
And then your support works,
And then tune the chit out of it.
When I did this; my 3650 pound 1968 Barracuda(367 cubes) went 106 in the quarter, smoked the tires to past 60 mpg with a 4speed and 3.55s, broke the tires loose at 40/45/50 in second gear (still 3.55s) , and on one point to point trip, made 32 mpgUSg; OOPs in overdrive.,lol, geared for 75mph =1830rpm
That's how I covered the bases.
You can't hit them all, quite the way I did with my manual trans, but you can sure have a lot of fun trying.
BTW.
I ran the A999 and 2.76s with my stock 1973 Smoggerteen winter engine, in this Barracuda, and it was dynomite big fun. The only bolt-ons were a 4bbl, headers, and a higher stall.
Oh Yeah
There are other roads to where you want to go; this is just the road I took... with no regrets.
I'm not trying to push you into this combo.
I'm just trying to give you an overvue of what is possible.
for instance;
If you already have a built 727 with a 2600stall and 3.23s,
and you got a lotta coin wrapped up in them,
then the combo I detailed is already all wrong, because 65=2610 @zeroslip, perhaps 100 to 150 more, depending on the load presented to the engine. It's gonna be hard to make fuel mileage with that rpm. Furthermore, 60mph is now ~3840 in Second gear, so there is no need for a 223* cam anymore, you can run a stock 318 cam and won't be much if any slower. And you don't need the hi-rpm Alloy heads either. You see the domino effect?
Happy HotRodding
