The numbers are on the right side of the trans case just above the pan.
If you don't know where to look, and your transmission is dirty/goopy with buildup, then these numbers can be hard to find. Follow the lip of the transmission, where it mates to the oil pan, along the outside. On the right side, there will be a flat machined area about 3 inches long by 1/4-inch high, and this is where the numbers are stamped. These numbers are less than 1/4-inch away from the oil pan.
Most of the time, the transmission number looks like "PKXXXXXXXX YYYY ZZZZ" where X, Y and Z are sets of numbers. Posting those numbers here, will help us identify the transmissions you have.
Unless your 400 engine was rebuilt and they decided to change the crankshaft, you have a cast crank in your 400 engine. The (balancing) weight on the torque converter is a little piece of flat steel, about an inch wide and 1/16-inch thick welded to the side of the torque convertor.
Around 1978, Chrysler Corp switched over to "locking" torque converters, to mechanically lock at highway speeds to improve fuel economy. These locking ones don't typically have a drain plug. There is normally a plug one can see from the front of the transmission torque converter. At 1977, you're right on the line, but if you can see the drain plug, you may not have a locking one.
Swap the transmissions, but save your older 1972-400 one. Swap the transmission and torque converter, together as a unit. If the 1977 transmission does not function to your satisfaction, you can have your 1972-400 one rebuilt, or do it yourself.
My first time rebuilding one, I was pretty intimidated and thought it would be more complex than an engine. Decades later, I have 6-7 of them under my belt, and agree they are pretty easy with a rebuild kit. If you're at all intimidated, watching a few YouTube videos will help. There are some decent books out there, too. I recommend the one by Tom Hand, although, it may have too many pictures, if you can believe it.
