Here is an excellent link where a guy describes how he designed his own doubler, and his drawings: http://www.bc4x4.com/tech/2000/gordtcases/gordtcases.cfm
With my NP435 low of 6.68
the Ford NP203 low of 2.01
the GM NP205 low of 1.96
and an axle ratio of 4.88
my crawl ratio will be 128:1 and I could have had more with the NP208 case
The Hummer crawl ratio is about what, 33:1?