Actually, there were 10 GACs built and installed in tanks. There were 6 M29 prototype tanks and 4 M32 prototype tanks. One was an up-armored version of a Pershing tank, and the other was a standard Pershing tank with a more powerful engine, because the V8 just didn't have enough power for the Pershing, which weighed 10+ tons more than a Sherman. So there were 10 prototype tanks with the 12. There's no record of how many spare 12-cyl engines were built. I know of two military parts wholesalers who have NOS carburetors for the 12s, so apparently there were some spares made. The Army overhaul manual for the GAA has a copy of the Ford dyno sheet, and it shows 500 hp @ 2600 rpm and 525 hp @ 2800 rpm. The overhaul manual instructs the depots to reject any rebuilt engines which do not dyno out to 475 hp.
I did talk to the military equipment collector who at one time had GAC engine #001. He had it on display with his collection. However, a fellow collector had one of the tank prototypes with the V-12, grenaded it, and prevailed on this collector to sell it to him for retrofit into his tank. I recently made a few inquiries with people who restore tanks to see if they knew of any of the GACs floating around, but nothing came of the inquiries.
In looking at the GAA cylinder heads, you can see the heritage of Ford's policy of putting severe doglegs in the exhaust. There would be a substantial power increase with raised ports. The intakes, as well, could have been much better designed. For no particular reason Ford elected to put one carburetor at each far end of the engine. Then they enclosed the intake passageways in the form of a cast-in, integral manifold. To minimize separation of the air and fuel particles, they used exhaust heat under the carbs, and cast in the hot water returns on top of the intake passageways. Despite this, the end cylinders ran very rich and the center cylinders ran lean. Nearly all used heads that I've seen have heavy carbonization in the end cylinders and valves, etc. The plugs that were used were inadequate, and often fouled. The plugs themselves were recessed in little preignition chambers which further restricted the weak spark. I'm told that each tank carried 100 spare plugs for when fouling occured. In my engine, as in all other modified engines that I've seen, the intakes are entirely revised to a contemporary design, opening the air passageways in the middle of the engine, plugging the ends, and putting the carbs (or blowers) right in the middle where they belong. It is my firm belief that there is 150 hp or more in just revising the intake side of the engine. I also think there is 50-100 hp in the ignition system, using MSDs with 6AL controllers. Opening up the precombustion area and using 14mm x .750 reach plugs now puts the spark right where it needs to be. You have to see the before and after to see where the HP improvement numbers come from. I think I'm realistic in projecting 250+ HP to be gained in updating to a modern standard, including using 89 octane ethanol instead of the 80 octane rotgut available in 1945. Far future plans include building a #2 engine with dual 460 throttle body EFI, (2) 30 or 36-lb injectors per cylinder and a Ford EEC-IV controller.