Inhabor, "slowing" down the TDI engine is important from at least two aspects.
1. With stock transmission gearing, the RPMs at 70 mph will be in the neighborhood of 3600. Unless you have extremely soft motor mounts and good sound-deadening, the noise inside the Bus will be very annoying.
2. Your TDI engine will be far more efficient and last much, much longer if the RPMs are below 3100 at 70 MPH.
I'm far from an expert on transmissions, but I did rebuild the tranny for my project. In fact, researching, rebuilding and re-gearing the tranny was one of the first things I did with my project.
I admit up front, I have very little knowledge or experience with pre-1980 Van/Bus transmissions.
My tranny is a 1980 model, first of the Vanagons, Code DK. It also has a 4.57 Ring and Pinion and come OE with a 0.85:1 4th gear ratio. I upgraded (taller) both 3rd and 4th gears. My 4th gear is a 0.77:1 as Markd89 referenced. I did the entire rebuild my self using over $1,000 in parts from Weddle Industries. Of course, I had to replace some damaged components which added about 1/3 to the cost. To date, I've driven it over 80k miles with no problems... shifts great.
I'm running 215/75/15 wheels/tires...
Take a look at the chart below showing Vanagon Transmissions. It doesn't apply to your Bus tranny, but it will give you some ideas. You can see the DK tranny and the original gearing and then at the bottom see my custom tranny.
Also, look at the chart, pick some ratios and apply to the Gear Ratio Calculator at Weddle
https://weddleindustries.com/
And, at this web site, you can plug-in your tire size to learn about diameter, revolutions per mile, etc....
https://tiresize.com/calculator/