You have the VF1A behind your R150f, which is a chain drive transfer case. The RF1A was only behind the 4 cylinder transmissions up to 95 and are gear driven (more desirable because you can lower the gear ratio). Here is a link to rockauto.com (remember VF1A behind 6cyl, last option is yours).
Excellent choice on the TDI. I plan on putting an alh into my 81 toy pickup after I graduate and start making some money. I have tons of threads saved on the topic if you were interested, although I'm sure you've seen most of them since almost all of the threads are on TDIClub.