Yup, its the output shaft cut and resplined.
I have no measurements yet on the length of the doubler/adapter plate but the shaft's lenght is about 8" and its built to the specs of the main doubler mentioned in the RRORC link above. The splines are 1.5" long and this is the correct length (according to the RRORC thread). But to get the adapter's lenght, you just stack the doubler case on top of the normal case (with the shaft in between) and measure the gap -- that'll give you the length of the adapter plate (plus a small fraction because you dont want the doubler's shaft to jam in all the way).
If you want exact measurements, let me know and I'll go get my caliper.
As for the transfer case I'm going to use, I was going to use my backup manual 1354 but I'd rather not slice that up so I think I'm going to pull a manual 1350 from a junk yard Bronco since the 1350s are a lot more common than the 1354s.