Nick,
Gear Vendors units are great, especially for sticks where you actually get a hi-lo for each gear if you want it. Same applies for the auto, providing the converter locks up in the intermediate gears. The big thing to watch though, is your power band. Like Bob has pointed out, his new engine has a power band that is considerably higher than his 383 was, and his OD is virtually useless at legal speeds. An OD might put you too low in rpm's, especially if you go back to taller tires? IMO you really need to settle on what size tires you're going to run, then pick your rear gear ratio to best match your rpm band and with that info decide if you want an OD gear. The only real advantage OD brings you is cruise economy, and running at lower rpm on the highway (which is generally desirable, depending on your torque curve and staying in your power band). The choices on tire diameter, rear gears, OD gear, and acceptable cruise rpm are all inter-related.

I can't speak to the dimensions of the various auto's, and what fits or what doesn't. I'm running a Tremec TKO600 with the 0.64 OD, and I dearly love having the third pedal.