Great Transit build and cool to see what you are coming up with.
The FrostKing self adhesive foam duct insulation is a good alternative to Dynamat, etc. But it's not R3. Closed cell foam is typically about R3 per inch so with a product thickness of about 3/16" your not getting a lot of thermal insulating. We used it under the cab floor in our Sprinter but don't recommend it for under the floor in back. It's not resilient and supportive enough and will compress considerably over time. Minicell closed cell cross-linked PE foam is better (used under dance/gym floors) and probably less expensive as well.
We would be honored to supply some 3M Thinsulate(TM) for your walls, ceiling and doors. You will be amazed at the performance and there is no need for any additional noise control. It's actually only marginally more expensive per sq foot than the FrostKing and will do a lot more for raising the comfort level in your van.
All the best,
Hein
DIYvan.com