I looked into this a while back (9 months?) and couldn't find anything. Part of the problem is the gears are so damn huge there isn't room. Have you tried posting this to the Chevy page? Just out of curiosity, why not the doubler with the NP203? What are you running for a tranny? I hear it's only 1" longer than the SM465/NP205 setup and the NP203 is a decent transfer. BTW, if you do find anything, let me know. I swapped in an NP205 and would love to have some lower gears.
JEEPN
'97 TJ Sport
'81 CJ-8 Scrambled!
'71 Commando SC-1
'51 CJ-3A
'47 CJ-2A