For super strength and durability the TH400-to-NP205 package is ideal. The long female input shaft and six-bolt circular mounting surface make for a great package, but we decided to do things differently. Since we wanted to use an Off Road Design Doubler between the two, this unit shown wouldn't work. It has a long 32-spline female input, and we wanted the shorter one with the eight-bolt racetrack pattern mounting for the most compact package.
The more common 205 case for GM trucks is the one with the TH350 bolted to it, which has a smaller input-bearing diameter than the TH400 application. Fortunately, you can simply bore the opening out to the larger size, if you have the right equipment. Ed Hotard of Performance Gear & Axle has done plenty of these conversions, and dials in his monster mill for precision machining.
Once we figured out all of our components we wanted to use, we laid them out for this photo. Input, low range gear, rear output, cluster, front output, low range gear. Notice that both clutch sliders and low range gears are identical, just on opposite sides of the case.
The center cluster or idler gear is supported on both ends by tapered roller bearings. This is far better than needle bearings due to the helical gears' inherent thrust loading. The bearings are adjusted by shims, and the oil flow is through the holes in the spacer between the bearings and the gear and shaft. Make sure all holes are free of obstructions, since this is the most important area for oil flow.