Why would you want to do this? There is no benefit (to my knowledge) in this conversion! The BW35 box is easier to source, replace, rebuild and or upgrade to BW40 or 51 specs, well above P5B specs. It will also draw off less power than the DG box.
I presume you're wanting to upgrade the engine but not the original trans to your P5?
If the engine is the mk3 version then the BW35 is a direct fit...this is how they came from the factory but unless you're mechanically adept (can weld and fabricate) you'd also want the other pieces from a mk3 to simplify the installation.