I do not have a 400 so much of what I will say will come from what I have learned coming from a farm and from getting one for my 318 that I am installing a rear sickle mower.
I use the John Deere parts to help in understanding what and how the drive shaft was used. The drive shaft you need to connect the rear PTO unit to the shaft located behind the pivot for the front axle is no longer available from Deere.
However it is available at www.drivelines.com
. Look at the L600 / 6 Series Driveline Parts. I believe this is the series John Deere used in the early years. They now use a lot of metric drivelines in the big batwing mowers and other equipment. Be careful of eBay most of them are metric or you can't tell the size of them.
You will need a quick connect yoke to connect to the tractor, a round bore or a spline yoke to attach to the shaft of your rear PTO, there were two versions. You also need male and female half shafts and the cross bearings (2).
You can also use PTO Application Data Sheet on their site to show them what you need and fax it to them or email it. They would get back with a recommendation. If you have not worked with a PTO shaft, this might be a better way.