The od box has ratios of; 3.09-1.67-1.00-.69od. If it has 2.76s, and 25.5" tires, then the final drive will be 1.90 and 65=1630; do you think your slanty can pull that? Even if it can it's gonna take a lot of throttle. The Vcan is probably gonna kick out, and at 1650 your mechanical timing is gonna be around 16 to 20 degrees, while the cruise timing is gonna want to be about 40. And there is not much you can do about the shortfall. This means any economy the 1.90 gearing might have the potential for, will be negated by the lack of timing.
3.23s are not much better, the final drive being 2.22, and 65=1900; you still can't give her the timing she wants with the mechanical distributor. The good news is that at the higher rpm the slanty will have the power, and maybe the Vcan won't kick out all the way. So it will be better, just waaaay short of great.
The problem with the 3.09od-trans is the rpm drops on the shifts. The splits are .54-.60-.69
So if you rev first gear to 3000, still with the 2.76 gears,then when she drops into second, the Rs will be 3000 x .54 =1620@27mph. So, now your slanty is gonna have to have some balls to keep on accelerating. This gear is like second and a half with the regular A833. To continue; the slanty will now pull to 49mph@3000. Then on the shift into third(direct) the Rs will fall to 3000 x.60= 1800@49mph...... and it's a long long way to 65mph @2370 in Direct. To recap, the speeds at 3000rpm will be; 27/49/82.. So in other words, to accelerate briskly, you may have to increase the Rs in each gear.
2370 is a great rpm for the slanty to cruise at, cuz you can set up the D to give her up to about 50* cruize timing, if that is what she wants. But at 65, shifting into OD, chances are you will get better economy in direct than at 1650@65 in overdrive.
Now, if you ask me, that overdrive trans might like a 3.73 rear gear for your slanty.
Your starter gear will be a nice 3.09x3.73=11.53 so taking off is easy..Now, 3000 will get you 20/36/61 in third; still long pulls, but you can't do anything about that. But, the slanty is powering thru the gears quite a bit quicker, and your cruise-rpm will be 65@~2200.
The nice thing about this 3.73combo is cruising at 30 mph in second gear is about 2470, versus 1825 with the 2.76s..... so keeping up in traffic makes for less backshifting.
So what about the 2.66 box?Well, with the 2.76s, and still the 25.5s;
65=2370, a very nice place to be for the slanty, as already mentioned. The ratios are 2.66-1.92-1.40-1.00 and the splits are a much nicer .72-.73-.71 and 3000 will be; 30/41/57, and 30 in second is 2100. This is a bit low, so not as much fun as the 3.73od combo. To match that 30=2470, your 2.66 box would need 3.23s, and your cruise rpm would jump to 2770@65mph. At this rpm, you can probably feed that slanty up to 54* timing if she wants it... and then you can lean out the carb, and get back some fuel economy that the extra 300rpm stole.
Sooooo, what about the 904, 3.23s and a Hi-stall TC? Glad you asked;
The odbox/3.73 take-off ratio above was 11.53. Say you revved that slanty up to an rpm where the engine made 125 ftlbs and slipped out the clutch and the engine delivered that into the clutch,and that got multiplied by the 3.09x3.73=1157 ftlbs by the time it got to the rear axles. Let's just say that rpm was 2600.
Ok so now swap out the clutch and install the 904/ combo. Again at 2600stall now,125ftlbs out to the back will be only 989 by math. But at zero mph, the TC has it's own torque multiplier ratio, which could be as high as 1.8 or more. So at zero mph you could have 1.8x989=1780 ftlbs........... compare that to 1157. Hang on now, this multiplier only lasts for a moment. As the car begins to move the multiplier diminishes and soon settles down to between 10 and maybe 5%. At plus 10% then, your 989 turns into 1088, and this is 94% of that 1157 best. That's pretty good.
In second, the od/3.73 makes a 6.23 ratio, while the 904/3.23/1.1TC makes 5.15, so now we are down to 83%, and falling behind.
Ok but I get that you are trying to make do with what you got. So IMO, if the 2.76s have to stay, then you need to run the 2.66 box. The road gears then (rear gear x trans gears) with the 2.66box/2.76 gears, will be;
--------7.34-5.30-3.86-2.76.. Now with the 904/2.76s/ and 1.1 in the TC, we get
10.14-7.44-4.40-3.04-2.76 not quite as good,lol, but say you just get some 2.94s; then..
10.81-7.92-4.69-3.23-2.94 .. You see what I'm seeing?
The first number you see there in the 10s, are with a TC multiplier of 1.5@ zero mph.I chose 1.5 for a stock-type lo-cost TC. The rest are with the multiplier of 1.1. And the final number is the rear gear at zero slip. This number at cruising speed,can be higher or lower depending on how hard the slanty is working. 2.94s will cruise at 65~2520(zero-slip).
What I see is that your automatic is really behaving like a 5 speed. And with the addition of 2.94s, it is blowing the 2.66box/2.76s away.
If you throw a hi-stall at that combo, it will do even better!
Now here is the clincher;
With a hi-stall, you might be able to throw a whack of timing at her, without running into detonation below the stall speed. So like with say a 2600, you could have 16* at idle increasing to 26*@2600, plus 20* in the can for 46* right there to cruise with. That will help the slanty make some torque and so you may be able to reduce your throttle setting and lean her out. And making more cruise-power with less gas, means "magic"; better fuel-economy..
Ok so the point I'm working up to is that, IMO, the slanty needs the 904, if you just think of the entire combo. The only thing better would be an A998 with a loc-up TC and the 904 gears: IMO.
BTW, with a hi-stall, you might even be able to leave the 2.76s in there. Compare the numbers; up to the top of first gear, say 4000rpm@41mph, the 904/2.76 is besting the 2.66/2.76s.. What about the od box you ask? Well, with 2.76s I get;
.......8.53--4.61--------2.76-1.90 compared to the 904/2.76s
10.14-7.44-4.40-3.04-2.76 the 904 wins again. Splits here are .73-.59-.69-.91 sweet!
Unfortunately, at 30mph this 904/2.76 combo is idling along at a tic under 1600 in second gear, which could slip to 1740 as you press the gas. Not very lively. So she may need to be downshifted, hitting 2950, well that's not so bad. I wouldn't like it much, tho.
Bottom line is; hi-stall and more gear..... IMO
