xram53,
Welcome to the 'bolt. Most of your questions will be answered, many with multiple opinions
I know from working on and researching my 53 3604, that there are several differences from the 49-52 ADs and from the 54s. More similarities overall though. Get the repro copies of the 53 Engineering Features (I think, my books are in the garage)and the 53 Owners Manual from any of the vendors. A very helpful group of publications are the Service Bulletins, available in repro for each year. For complete coverage of a 53 model year you will also want the 52 and 54 series as well. The end of the 52 series (Nov & Dec) discusses engineering changes and new features in the upcoming, 53, model year.
Even for your 6400 you should be able to get most parts either in repro or used.
Keep the questions coming and keep on truckin'
