Overdrive in Early A

If I was to do it in an early-A; I wouldn't.
If that statement irks you, read no further.

What I would do and have already done is;
I'd just install an engine with more torque and gear it to fit my needs.
Part-1 of the reason, is this:
it is impossible, with the factory-type distributor, to give the engine, any mopar engine, the optimal cruise timing it needs below about 2400 rpm, and still have the Power-Timing set right.
The optimal cruise-timing n a Lightweight-A, for 65=2400rpm, is gonna be about 52>56 degrees, or more.
To get to a Power-Timing of 36*@3600rpm, your Distributor will need a timing curve of about 1.0* per 100 rpm beginning at 1000 rpm, and an initial Idle-Timing of 10 degrees.. So at 2400 it will have ~24* total mechanical. The VA can be modified to about 22*, bringing the total cruise timing to 46* at 2400 ..
Now, you can maybe cheat a lil with a two-stage curve, and pick up maybe 3>4 degrees at 2400 for a total around maybe 50*; but still not 54*
Or maybe you can over-advance the Idle Timing to say 18*, run into detonation and have to slow the top end down, and get maybe 52..
But to get to 54/56* is impossible. Driving any slower is only gonna make the fuel-economy worse. So I see no point in cruising any slower, if you have a Factory-type distributor.

My car cruises at 65= 2240, but it cannot make optimal timing, so the fuel economy gets better with a li more speed. 2400 is 70mph.
To solve this, I have installed a dash-mounted, dial-back, timing device with a range of 15 degrees, so I can, find the perfect timing, right from the front seat, for any circumstance.
If you try to run 65 below 2400 rpm without optimizing the timing, the fuel economy will suffer, I can guarantee it.
On the other side of the coin,
Part 2
running an overdrive of .67 to .72, is about the same as having another gear; which means, to cruise 65=2400 rpm, you can run 3.91s.
Well, the 4L80 has a first gear of 2.48 and so, your engine will deliver, to the rear, whatever torque comes out the crank x 2.48 x3.91.
Say you have a small engine and a stock stall around 1800 rpm.
Say 200 ftlbs comes out the crank at 1800. Then
200 x 2.48 x 3.91 = 1939 into the rear axles. So then that is all she wrote.

Ok lets swap a bigger engine in, run 65=2400 still, but no more overdrive, The 2400rpm cruise means you gotta run a rear gear of 2.76; but you still want at least 1939ftlbs, as before. So then, lets swap to an A999 with a lock-up and a 2.74 first gear.
1939/(2.74 x 2.76) =256 ftlbs required from the engine, which is ;
(256/200) less 1=28% more torque, so you need to pump up the 200ftpounder to that, or just slip in an engine that much bigger with the same build spec, or some combination of the two. and thus your car will be the same quick, off the line, with 2.76s and the bigger engine, as it was with 3.91s and the smaller engine.
But wait; there are 4 small-blocks to chose from and hundreds of ways to build them, to meet the goals of 65=2400rpm, and still make 1939 ftlbs on the start line. So you don't have to stick with such a modest amount of torque at zero mph. I mean even 4000 ftlbs is possible, with a relatively modest SBM.

Okay, I'll bet your car already has 2.76s and a 904, so it's a natural for a modest 360, which can easily make 300 ftlbs at 2400. Out the back door, that would be, say 300 x2.45 x 2.76 =2028 ftlbs, already more than the above example of 1939ftlbs. and it's a stock 8.0 engine!
Say you chose a 5.9Magnum, I bet she makes 350@ 2400, now yur looking at; 350 x 2.45 x 2.76= 2367!
Lets put an A999 behind that 5.9M and add a 2800 stall. Now she is up to 380 x 2.74 x 2.76 = 2874 ftlbs. and still cruising at 65=2400rpm plus slippage, say 2500 on the tach.
I have done this with a fully stock long-block, lo-compression 318, an A998 (no LU), a 2800 stall, and 2.76s in the heavier Second gen Barracuda, with excellent results.
Well ok, I installed a 4bbl on it and headers with dual 3inch exhaust, but that shouldn't affect it's economy much if at all, assuming I kept my foot out of it, which is always hard for me.
Now, some guys on FABO will remind me that 3.91s will wind thru the gears faster, and so I'm out to lunch.
Well I won't argue that, but unless you already have the Biggest small block under the hood, so what.
With the little engine, with the 4L80, and gears of 2.48 x 3.91 at will hit 5000 rpm at about 35 mph and when it shifts, the Rs will fall to 2980 and dog-city. Whereas
with the bigger engine, the A999 and 2.76s, 5000 will get you 45 mph before the shift, while the lil engine is just coming up to speed at 3800 in 1.48 Second gear.
So I mean, draw your own conclusions, about that.
My conclusion is that, with 2.76 gears; the 5.9 Magnum/ any TF trans with a LU convertor, will/should, fit the bill. Failing a 5.9M, even a a small-cam 340, or a hi-compression 318 with a 2800/no LU, should fit the bill, allbeit with a cruise-rpm about a 100 rpm higher.
If I was forced to use a 273, I would run a small turbo on it, just to pump up the low-rpm torque, and run any trans with still, the 2.76s..... cuz 65=2400 is pretty sweet, and I can't run big timing anyway with a turbo.....

What I mean and am trying to say is, installing an overdrive into your Early-A is a pretty big deal, and I'm just offering an easier and probably cheaper alternative that can see you running down the road in a matter of days;
Not months or years. Ok so
Happy HotRodding