You can't use the trans adaptor plate with the ZF6 and without that plate, the top two bolt holes are not going to bolt to the block. Plus, like IACA said, the fluid cooler, flywheel and clutch would have to be a custom deal. JayBee was the member who did the swab on his IDI, but he's a professional fabricator, not your average do-it-yourself'er.
Plus, all that work and you really only gain a granny gear. Ever see the pattern at the top of a ZF6 shifter? L, 1, 2, 3, 4, OD. That's a ton of work to get a "low" gear under 1st.
Heath