Here's my kick at the can.
The 5.2M has plenty of torque in stock form. At least plenty enough for what you want to do. Therefore, you don't need much TM(TorqueMultiplication) help from the convertor, Therefore, you can pick a rear gear, that gets your cruise rpm down low, but high enough to be above intake reversion, which should be easy with the cam indexed properly. My guess is that this will be about 2000/2100 rpm. I can show you how to figure that out exactly, see note-2. below.
Lets say yours is at 2100.
Here's the deal, there is NO WAY to give your engine optimum ignition timing at 2100, with the factory style timing advance mechanisms; so yur gunna need a timing computer. I can show you that too, see note 1.
After that, yur gonna need a carb with metering rods, to dial in the cruise AFR.
That will get your cruise optimized.
Now
to get it off the line and moving briskly, yur gunna need a modest stall.
To maximize your fuel economy, the LU is a good idea, as is the 2.74 low gear.
The lower starter gear is worth TWO rear gear ratios LESS than with a 2.45low in the regular A904.
The LU is less valuable in your case, with a stock stall, you might get 100 rpm reduction out of it.
So now, let me lay it out for you;
Get the A998, low gear, non-LU trans
Install a 2.76 or 2.94 rear gear.
Get a 2800 stall.
Get the Cruise timing right with the computer.
Lean the crap out of the cruise AFR..
Forget about the loss of the 100rpm from not having a LU.
Here's how this plays out;
The trans has ratios of 2.74/1.54/1.00
The 2800 will stall at near 1.8 to 2.0 TM.
Your Dart probably has tires of a tic under 26" tall. Lets make it a runout of 81 inches., And so, 65=2500 with 2.94s, or 2300 with 2.76s
Lets install 2.76s, cuz there is a cheap set in nearly every Mopar guys garage, with no home. So that's it we're gunna optimize the engine for 65= 2300.
You're gunna install your intake and 500 carb.
First gear is gunna take off with the starter of 2.74 X 2.76= 7.56, which you know is gunna be lame...... if it was a manual trans; which it's not.
The 2800 TC is gunna have a ratio of about 2/1 at zero mph, so then, that 7.56 is gonna become 15/1 for a few milliseconds. and at 2800, your Magnum might put out 250 ftlbs, so that would be 250 x 15= 3750 ftlbs into the rear axles. Get a posi, cuz she's gunna need it.
Now, as soon as the vehicle starts moving, that 2/1 ratio, is gunna diminish towards maybe 1.4 at the top of first gear. which would then be 2650ftlbs to the road; still pretty healthy. Now, at 4500 in 2.74 first gear, your speed is approaching 42mph, so yur all smiles and lifting off the gas. Meh, 5000 will be 47mph.
Ok so, that's all she wrote for performance.
Now back to economy.
Cruising at 2300 , your engine will want more than 50 degrees of timing, I can practically guarantee that she'll want up to 56degrees.. How are you gunna get it?
Typically, from the mechanical system, you'll get .8 degree per 100 rpm beginning at or near 1000 rpm, so that comes to 10.4 degrees, at 2800.
Lets say you give your automatic-equipped engine a chitload of Idle timing, and it doesn't make a tip-in sag; say 18*.
And let's say you spend three days looking for a VA with max timing of 14*.
Total timing is thus ; 18+10.4+14= 42.4 and yur a good 10 degrees shy.
This is what's gunna break your fuel-economy attempt. You need to get the potential up to 56, and you need to get the Idle-timing down closer to 14*.. Yur gunna need a dash-mounted, dial-back Timing-module, like mine, with a range of 15 degrees; and yur gunna need to modify your VA for about 22degrees.
The total now is say 12 + 10.4 + 22= 42.4, plus with up to 15 in the Dial-back, is up to 57.4, now yur talking.. Now you just, by empirical testing, figure out what your engine wants, and set the DB to that number. Easy-peasy. Just don't forget to dial it back after coming down off the hiway.
and then, you start playing with that AVS-2. You will be surprised to learn how lean you can run when the timing is correct. Yur may need a selection of jet-needles and one or two pairs of smaller Primary jets.
Ok so, notes;
Note-1
How to determine your best cruize timing;
With the VA hooked up to intake manifold vacuum,
Rev the engine up to the desired cruise rpm and fix it there.
Add 5 degrees of timing. If the rpm went up, put the rpm back, then do it again. and keep doing it until more timing does NOT produce an increase in rpm, then take 3 degrees out, and put the rpm back.
Now, read the timing. Write it down before you forget it.
Idle the engine down, and return the timing to as you started with, then let it cool for a few minutes.
What number did you get?
Whatever it was, that is gunna be darn close to the perfect cruise timing; only testing will prove it.
Note-2
How to determine your maximum cruse rpm;
Plumb your VA to the intake, so it sends it's advance.
Run the engine up to 1600 rpm, and measure the engine vacuum.
Increase the rpm to 1800, and measure the vacuum.
Keep repeating for as long as the vacuum keeps increasing. When it stops, go back to the last one that increased. That was the first rpm where efficiency was at it's best. From there you can optimize further with Timing.
If you cruise at a lesser rpm, the engine will lose efficiency about as fast as the reduced rpm gains economy. If you keep reducing the cruise rpm, with no other changes ; loss of efficiency will likely outpace gains from reduced rpm, and so, she will start to lose fuel-economy.
Note-3
This is a tried and true recipe, that got me 32 mpg from a 330hp/360 manual trans, cruising at 75=1850 rpm. Car ran 106 in the quarter, with the only change being a 750DP.
Trans was, at that time, an A833od with a GVod strapped on the back, and 3.55 gears.
