do I need an adapter plate?How do you "cant" the motor in, is that part of the adapter?
"Cant" is just another word for "tilt." Usually it's more an architectural thing than a common usage thing. Just me showing off my vocabulary, don't worry about it.

Yes, you'll need an adapter plate no matter how you mount it. The Aircooled flat-four bolt pattern and the watercooled inline four pattern are not compatible.
I believe that Kennedy has a Vanagon-angle adapter plate that puts the engine in the appropriate position. You'll then need the oil pan and pump from a Vanagon, and probably the injection pump if you're going TD, so it will all fit under the decklid.
You'll also need to fabricate some way to provide support for the engine, like the Vanagon did with the side bars. Most T3s relied on the front transmission mount and the rear engine support bar to hold the engine/transaxle assembly in the car. Pre-'69 cars had swingaxles, and had horns on the subframe to hold the transmission in the car, but it would probably still be best to support such a heavy engine independently of these simple supports. It does weigh a bit more than the aircooled, 60-80# more if I remember correctly.
Speaking as an owner of a "humped" Squareback... it's more than just aesthetics. Building a box over the engine bay very effectively
slaughters your ability to load the back of the car. Not only can you no longer simply slide things in and lose a little height, the space to either side of your box is essentially useless, and removing the decklid becomes an exercise in restraint as you try to lift it and work it around whatever it is that sits above the lid level and required the box to be built in the first place. There were times I drove around without the engine lid at all, simply so I didn't have to deal with that hassle. Exhaust smell finally drove me to put the lid back on, but I was willing to deal with the noise and some exhaust gasses to avoid that nastiness.
When I was contemplating my own conversion of to diesel in a Square, I decided that the best option for dealing with such issues and not losing my mind on a regular basis would involve getting a rusting-out donor Square, cutting the rear cargo floor out, and raising the entire area around the engine by the height of box needed. No more box, but a whole host of new problems, fabrication, and opportunity.