What is the best way to get an overdrive auto in a 67 Barracuda?

Lets say you have a hypothetical engine that makes 375 hp. This is enough engine to go 108 in the qtr.pulling 3675# This is a nice easy 360 build.
The peak power might come in at 5600, and with good heads can rev out to say 6200 before dropping off.

ComboA)Let's say you had a regular 904 with ratios of 2.45-1.45-1.00. The splits are .59-.69. So the rpm drops will be to, 3558 and 4278,for powerband requirements of 2642/1722, or about 2182 average. So you have to put the right bolt-ons to make that happen.
To trap well,say at 5800 plus slip, you would need about 4.30s. These gears will get you a starter gear of 10.54, and 65=3472 plus slip.That's what it will take with the 904.

Combo B)Now, let's work it out with the 4L80
The ratios are 3.06-1.63-1.00-.70 and splits of .53-.61-.70od. Let's say same 6200 rpm shifts; the drops are to;3180 and 3660, for a powerband requirement of and 2820/2540 or about 2680 average.
To trap at 5800 plus slip ,will require 4.30s.
The starter gear is 13.29, and 65=2430plus slip.

ComboC) Finally lets put a GVOD behind an A999, and split gears.
The ratios are 2.74-2.14-1.54-1.20-1.00-.78od, and splits of .78-.72-.78-.83-.78od Same 6200 shifts; the drops are to 4836-4464-4836-5146-4836 The powereband requirement will be from 1488 rpm
To trap as before but now in second over,ratio 1.20, will require a rear gear of 3.55s.
The starter gear will be 9.73, and 65=2236

Now compare the numbers and select a tranny based on your useage.
Obviously a street/strip car would like either of the second two options rather than Combo A).
But which one?
Well Combo B) looks pretty good. But I can tell you two things. 1) the 13.29 is a ridiculously low street starter gear, and 2) the powerband requirement of 2680 is still pretty wide.
Combo C) cures the starter gear very nicely, and with a 2236 cruise rpm, that's nice also. But best of all is the powerband requirement of just 1488rpm.
http://www.hughesengines.com/Upload/TechArticles/land_dyno.jpg
Take a look at a typical dyno graph. On this one the average hp over 1488 rpm centered on the peak, is about 420hp. Now look at it again with a 2680 average PB; does it look like around 345 hp average?
Which tranny do you think will be faster, and which one are you liking.

And BTW, you could gear combo C) up 20% to 4.30s like the others and take advantage of the short 5thgear. The new starter would be 11.78, and 65=2708. This would tighten up the powerband yet further. . Furthermore, the very small difference between the peak power and the average power, would allow you to detune the engine one cam-size for sure, maybe more: and still go as quick or quicker as the 4L80.

The point is do not underestimate the transmission. The right one can turn your mild streeter into a dragon-slayer;the wrong one into a turd.