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

The neat thing about the GV, is you can use it as a splitter. You can make your 727combo into 6 useable gears. GV in red;
2.45-1.91-1.45-1.13-1.00-.78od, with split percents of
.78-.76-.78-.88-.78.
If you gear your car to trap in 2nd/over and skip direct completely, then the 2od-3od split is .78/1.13=.69
Examples;
without the splitter;
say you now trap at 118,in direct with 4.30s at 6400,having used 3 very wide ratios, and two tugs on the stick. Your starter gear is 2.45 x 4.3=10.54, and 65 will be about 3354/zero slip
With the splitter;
you can use 3.91s, and trap in 2od at about 6600, but having used 4 very tight ratios,and only pulled the stick one time.The starter gear might be a little steep at 3.91 x 2.45= 9.58,And 65mph will be about 2479rpm, with zero slip.
The average horsepower for the GV run will be up much closer to your peak horsepower, and your shift rpm can be lower too.

For guys that can use the A999, the ratios are
2.74-2.14-1.54-1.20-1.00-.78od and the splits;
.78-.72-.78-.83-.78
This tranny is a gem
Again trapping at 118 in 2od, this time with 3.73s, the Rs will be about 6660.The starter gear is 3.73 x 2.74 =10.22,and cruising at 65 will be about 2269 with Zero slip.

That's the math of it. I used 5% TC slip at shift-rpm
IDK if GV has a 904 adapter.