The reason for the low 1st gear was that a stock VW is about 900 lbs. heavier than a VW based MGTD kit.
I used a Stuff Transaxles “Freeway Flyer” transaxle in my car. It did nothing for the lower gears, but made 4th gear higher so that I wasn’t turning high RPM’s on the highway.
Try calling Stuff Transaxles in Oceanside, CA. He is a VW transaxle Guru named Hank Harris. http://www.bugtrans.com