Looking for an anto od trans for a 66 /6 Valiant

What are you trying to achieve?
I mean there are a few reasons for wanting to do this kind of swap, and some of them do not require an overdrive.
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As for myself for example;
If I wanted a lower cruize rpm and also did not want the manual-trans any more, But I didn't want to lose first gear take-off power, here is something I would consider:
Overdrive ratios usually come in at around .69 to .78; which is an average of ~.74. Lets say that your rear gear is a 2.94, and your tires are 24" tall. This would make your cruise rpm to be, 65= 2680. Thus, with overdrive, your rpm will be reduced to; 2680 x .74=1980.. With an A500, actually to 1849rpm
So the next question is; will your /6 pull that? and the answer is, a 225 might, but probably a 170 will not. This means that you will need to change your rear gear up, until the slanty can pull it. The point is that to pull the A500 at 65mph, you will likely HAVE to change the rear gear as well.
>and the next question is; what will happen to your take-off performance.
Well, your A903trans probably has ratios of 3.22-1.84-1.00 so the starter is
3.22x2.94=9.46... But with the A904 auto, the ratios are 2.45-1.45-1.00, so the starter will be 2.45x2.94=7.20 but your convertor might give you 1.6 to 1.8 back, so
7.2x1.7= 12.24 which is 29.4% better than the 9.46 of the A903. Which means, that you can swap out the 2.94 gear for 29.4 less gear and be no worse off, at the starting line.
Ok then 2.94/1.294= 2.27 gears.
But there is nothing like that in the early-Mopar line up. Well maybe 2.20s but good luck finding one of those.
But there is a 2.76, and maybe you can find a 2.45 from a later model car.
Let's say you consider a 2.76. Your new cruise rpm is 65=2510 at zero-slip,(say 2590 at 3% slip.) and say you score an A999 LU. the ratios are 2.74-1.54-1.00 and it has a Loc-Up, so your cruise rpm is 2510., your starter gear will be, 2.76x2.74x~1.7=12.86 still 36% better than with the A903. This indicates that you can give up yet more rear gear.

>Let's say you score a 2.45 rear, and still the A999LU. Your new cruise-rpm is 65= 2230, and the starter is 2.45x2.74x1.7=11.41; still ~21% better than the A903.
So now, what about Second gear?
>Currently with the A903 and the assumed to be 2.94s, the ratio is 2.94x1.84=5.41
With the A999, the ratios would be 2.45x1.54x ~1.3 in the convertor =4.90..... so, Second took a bit of a hit, about 9%. But-um First gear got plus 21%, so here's an idea,
>lets swap in an A998LU instead. the ratios are same as the A904, so 2.45-1.45-1.00, and lets go back to the 2.76s. The new roadgears are ;
11.49-5.20-2.76 compared to the current
9.46-5.42-2.94 See, there you go,
21% better first gear, about same second gear, 6% better hiway cruising.
The point is, that yes an overdrive would be the cat's meow, but depending on your current rear gear (I assumed it was a 2.94), you can make other combo's work, until the cruise-rpm gets too low for your slanty to maintain speed while cruising.

>>But, if you're looking for fuel economy. Firstly, every 1% reduction in cruise rpm is said to produce a corresponding increase of just a half a percent in fuel-economy. So then going from 65= 2680, to 65=2510, is a drop of 167 rpm, which is 6.25 percent, producing a savings of 3.1% fuel economy. If your current combo is doing 20mpg, then you are looking at .624 mpg increase to 20.6mpg. You'll never get your money back from the above swap, nor, more likely, from any gearing swap.

Now, getting back to what would I do?
For fuel economy with no loss in performance,
Hands down I would put a V8 into it with 2.20 gears, for 65= 2000. a drop of 853rpm, or 31.8%, and an economy increase thus of 15.9% from say 20 to 23mpg. Now that's a difference that counts. Sure first gear with an A999 is gonna be an abysmal, (that's a joke), 10.25 compared to 9.46 of the A903/2.94 combo, but criminy Ima putting a 2800LU in it, and we are gonna burn rubber!
If I'm going to this trouble for fuel economy, Ima gonna install a lo-po 273 bumped up to hi-compression, with a 4bbl. But more likely I'll be installing any old 318 with a "factory" stall LU; probably around 2400.
This swap is not complicated at all, but does require some forethought, and time to collect the parts, and a bit of cash. But the end-result is so worth it! I once did this with a hi-compression 340 bottom end, a factory 318 top-end, a 318 hydraulic cam, and a 2bbl!. In a 65 Valiant wagon, it was a torque beast, and was so easy on gas....... back in 1974 or so.

>In your case, since your car already has a clutch in it, I would just install an A833 overdrive. The ratios are
3.09-1.67-1.00-.73 od compared to your A903 at
3.22-1.84-1.00. The rpm drops from 65=2680 to 2072, a drop of 608rpm or 22.7%, for an economy increase of 11.35%, or from 20mpg to 22.7. This is a really good gain for low dollars. This is a win-win, almost zero loss swap. Almost zero, cuz it ain't an automatic, lol, and it will cost you a driveshaft-conversion to a slip-yoke; but the rear gears can stay. I have noticed that the od boxes are rising in purchase-price, so get one asap. I used to buy these at swap meets, late in the afternoon, for $50. But I guess those days are gone. You will need the matching BH/fork/etcetera, and a 4-speed floor-shifter.