I have done this twice.

First was with a 69 Barracuda fastback. 318 with 360 1.88 heads; hyd cam w/specs midway between stock 318 and 340 cams. 3.23 axle.
With standard 4 speed: 17-18 mpg highway
With OD4: 24 mpg highway.

Second was 69 Barracuda conv. Slant 6, stock cam, Holley Economaster 1920 1 bbl. 2.76 axle
With standard 4 speed: 19-20 mpg highway
With OD4: 27.5-29 mpg highway.

Now have 2.94 axle (2.76 was just too low with OD). Haven't done a long range highway test yet, but around town stayed about the same at 22-23 mpg.

Robert

AAR 4-speed 3.91, Tor-Red;
70 440 6 pack Roadrunner 4-speed 3.54, Plum Crazy;
68 Formula S conv 383 4-speed 3.23, Electric Blue;
69 Barracuda conv Slant 6 OD4 2.94, 71 B5 Blue;
78 Lil' Red Truck, Red;
70 Challenger S/E. 505 6 pack, Passon 5-speed, 3.55, B7 Blue