IIRC the manuals used in the vans were light-duty trannies...even if it were possible to physically hook one up to a diesel, I have a feeling the engine would kill the tranny in short order. Also, I could be wrong (I'm going off memory after having been up 24 hours), but FWIW I think the floor-shift manuals were connected to the tranny by linkage as opposed to a direct connection....maybe it could be adapted to a ZF5, if you could find the linkage setup?
I think a manual conversion in a van can be done, and I'd love to see it happen, but I have yet to see someone pull it off on an IDI. If you've got the pedals figured out, making the gearshift lever something you can realistically control will be the big challenge, methinks...