While the conversion to the 3.4 is relatively painless when you are converting from a 2.8, the 2.6 is a whole different animal.

A partial list to convert from the 2.6 would include the following:

The Transmission won't work. The bell-housings are very different.

The wiring needs major changes. The ECM is diffent. The primary wiring is different.

The engine mounts are different and located differently.

The cross-members are different.

Even the wiring for the secondary instruments is different.

On the other hand, you can use a rear drive-shaft from a 2.6 in a 2.8/3.4 (I am doing it).

For a very short time, many years ago, Advanced Adapters did produce an adapter from a 4.3 to a 2.6 transmission. The removed it from the market and destroyed the template.