Other than valve body calibration, the 400 hasn’t changed much since it first came out in 65. In 1969, Hydromatic changed the center support by machining an extra .040 off the case lugs. They added a .040 snap ring under the center support. This prevented the center support from wearing the case. In 1971, the desirable smooth race direct drum was replaced with the weaker roller clutch style. The later drum can’t be upgraded to a 34 element sprag The early style drum returned with the introduction of the 4L80 trans. In the mid 70s, the aluminum pistons were replaced with less desired stamped steel. This was a cost savings change. The aluminum accumulator piston in the valve body was also changed to plastic, they break and should be replaced. The aluminum pistons are getting kinda hard to find. The 4L80 bonded direct drum piston will retrofit into the 400.
Ideally, find a core between the years of 1965 and 1971. If the core is older than 69, upgrade to the later center support and include the .040 snap ring. If it’s newer than 71, used a 4L80 direct drum and install a 34 element sprag.
Some motor homes and big trucks used straight cut gears, those transmissions are known as 475. Some also use parking brakes on the output shaft and extension housings. They can be replaced with normal 4wd and 2wd output shafts and extension housings.
Hope that helps.
