Down here I've seen C4s and Toyota Cressida OD automatics. Flat attack down here also does a kit for a C4 auto. Here's the Toyota OD kit, http://www.flatattackracing.com/cressida.html PS Less parasitic drag with a Toyota when compared to an AOD, a C4 would be comparable to a C4 however you have OD with the Toyota auto.
Hard to beat the Ford C4. Unless you are in need of an Overdrive, then the Ford AOD works well if you have a well built flathead. Most stock flatheads have a hard time pulling an OD, especially in a truck with a less than optimal drag coefficient.
For a automatic behind a flathead you would be hard pressed to find anything that works better than the good old C-4. HRP
I am a big fan of the turbo 400 in a truck, but not behind a flatty. They take too much torque for a flatty to spin and still perform. In a flathead truck (any year) if I was going to use an automatic I would use a Ford C4. Easily adaptable to almost anything, lots of pieces to beef one up if need be, small light and not a resource hawg.
I'm a GM guy and a 400 fan but I'm also smart, if the Ford and flat head guys recommend a C4 that's what I would do in an instant. No sense in reinventing the wheel. Pat
Mine is a 1967-70 There are a few adapter kits on the market. I purchased mine from Speedway, $400. Others are up to $800. Check them out to see what suits you.
They have not really changed much since '64. I would look for a newer one just because it is more likely to have some life left in it. I had one in a '68 Torino that was about indestructible. I went to a big block and sold the Windsor and tranny to another guy who wore out a couple of small blocks with it.
A few things to think about. When you run the large heavy duty transmissions like a 400 Turbo or comparable, they will take a lot more HP to operate and a flathead normally doesn't have excess hp to waste. A three speed automatic will have a higher first gear, so you may need to use a lower geared rear end to have decent acceleration......not talking race acceleration. That will in turn affect driveability if you get on the xway. Nothing is more annoying than driving somewhere with an engine running a high rpm. An overdrive of almost any type will give you a lower (higher numerical ) first gear, AND an overdrive gear for the xway..........making your vehicle much more pleasant and efficient. You get better low speed acceleration and lower cruise rpms on the xway. Given the lower power numbers of a flathead I think the most enjoyable transmission will be a lighter duty overdrive transmission. I think one of the biggest decisions any builder makes is what transmission he puts in the car. Many people don't want to drive to shows because they don't have good over the road capability. Take your current daily driver and get on the xway and hold it in 3rd gear for a few miles and see the difference OD makes. https://www.speedwaymotors.com/Flathead-V8-to-Ford-AOD-Transmission-Conversion-Kit,25353.html Looks like an easy to make adapter or can probably buy just an adapter if you can get your own tranny.
I'm considering going down this auto road more and more and am curious to know who has successfully used the Ford AOD or the Cressida AOD and what chassis mods were required?
C4s in both the truck and FED. rebuilt right here in own garage.........did use Flat-O-Matics torque converter and adapter kits though.....