Dan,
The easiest way is to use heater boxes from a 411/412 engine: they have straight tubes where you want to shorten them. Also they don't have the aluminium finned block around the headers tubes like the stock type 1 and all bus (!) type 4 heater boxes have. The headers inside the bus heater boxes have a difficult routing to shorten to be considered.
That aslo means less heat, but it will mostly be enough IMO.
Then you have to shorten them about 8 - 10 cm. Just cutting and rewelding the flange will do. You also must relocate the risers on the heater boxes to which the hoses will be connected to just behind the flange. Difficult to describe, I guess...They are also for sale already shortened at
www.ahnendorp.com
Good luck,
Walter