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GM TH700 R4 / 4L60E Automatic Transmission:
The TH700 has the same bolt pattern on the back of the transmission as the TH350 and uses a 27 spline output shaft. Since the TH700 only came with the NP208 or later alum t-cases, it's shaft stickout is a little over 2" in factory form. One difference to be aware of is that the TH700 uses 4 metric bolts to secure the adapter to the transmission case.
There are two ways to bolt to the NP203. They are very similar to the TH350 mounting mentioned above.
1: replace the output shaft in the TH700 with an aftermarket output that sticks out of the case by about 7/8". This makes the output identical to the TH350 and you use a TH350 version NP203 with the 27 spline input and a TH350 to NP203 factory 1" thick adapter plate. This is the shortest method. The output shaft cost is $160 and it is available from ORD.
2: use the factory TH700 output shaft and the factory TH700 to NP208 adapter housing with a TH350 version NP203. You will have to re-drill and tap holes in the NP203 gearbox to be able to bolt it to the original NP208 adapter housing. You may also have to do minor machine work on the input bearing retainer on the NP203.