Adapting the TH400

GM TH400’s primarily used 3 different output shaft lengths, all are 32 spline.

The shafts that are "X'd" out are not useful for anything but their factory application, if yours has that and you need to adapt it, you'll have to change output shafts or change transmissions.

To adapt your TH400 to a transfer case system, first you'll need to determine which output shaft you have. 

Output sticks out ~1-3/8” from the end of the transmission case:

This is the factory output shaft for TH400/NP203 systems.  These are found in ’77-79 Chevy K20 and K30 trucks.
 

How to use with Offroad Design 203/205 doubler: Use factory TH400/NP203 adapter, those are found in '77-'79 K20 and K30 trucks and are also available new HERE.
 

How to use with Offroad Design Magnum underdrive system: New 1” thick adapter from ORD HERE
 

How to use with NP205 alone: Does not work

Output sticks out ~2-1/2” from the end of the transmission case:

This is the factory output shaft for all TH400/NP208 and TH400/NP241 systems as well as TH400/NP205 systems from ’85-90.

How to use with Offroad Design 203/205 doubler: Use TH400/NP208 or TH400/NP241 adapter HERE.  Drill/tap six bolt transfer case pattern into face of NP203, plug up stock holes and make a tab to hold down the idler shaft.

How to use with Magnum underdrive system: Use TH400/NP208 or TH400/NP241 adapter HERE, the Magnum box bolts directly to either of those.

How to use with NP205 alone: Use long 32 spline input gear in NP205 HERE in conjunction with the TH400/NP205 adapter that matches the NP205 bolt pattern.

Output shaft sticks out ~4-1/2” from the end of the transmission case:

This is the most common TH400 output shaft and came in most 2WD TH400’s as well as ’84 and older TH400/NP205 systems.

How to use with Offroad Design 203/205 doubler: Use new TH400/Ford NP205 adapter HERE from ORD or rare TH400/round pattern NP205 adapter , Drill/tap six bolt transfer case pattern into face of NP203, plug up stock holes and make a tab to hold down the idler shaft.

How to use with Magnum underdrive system: Use new TH400/Ford NP205 adapter from ORD or rare TH400/round pattern NP205 adapter, the Magnum box bolts directly to either

How to use with NP205 alone: Use short 32 spline input gear in NP205 HERE in conjunction with the TH400/NP205 adapter that matches the NP205 bolt pattern.