-Your upper mounts will work, your trans mount will work, I do not think the passenger side engine mount on the L26 will not work.
-the L26 has an aluminum oil pan too IIRC. this will not fit in the 96 engine cradle and the 97-08 GP cradles will not fit in your 96. So the Oil pan will need to be changed to one from an L36. or you will need to cut, weld, and reinforce your cradle.
-the 08 has a returnless fuel system. your pump is not compatible with that system, nor are the fuel lines. It might be possible to run a L36 fuel rail but im not sure if the 08 PCM will properly control the older style pump. even if you get that pcm to control your current pump, the fuel line for your 3100 connect in different locations at the fuel rail then a L36 or L26 so these will need to be changed as well.
-The down pipe from the 08 will not fit. the engine bay in your car is smaller then in the later GP's. I do not believe ZZP even sells a DP that will fit a 3800 in your year car. Milzymotorsports may have one but it will be pricey.
-the axles you need are from a 98-99 Z34, LTZ, Police Lumina. these are the correct length for your car.

Now on to the electronic issues...
If you try and run an 08 PCM and harness and mate up to your 96 car here are a few issues you will have:
-the C100 connectors at the fire wall are different shapes.
-the 08 PCM will be looking for the BCM that will not be there.
-your instrument cluster is not compatible with the signals the pcm is sending. so you will need to swap clusters.
-the drive by wire gas pedal most likely will not mount to your pedal box on the fire wall.
-if you need any tuning or adjustments to the PCM to make things work, you will not be able to tune the 08 pcm.

Im sure there are plenty of issues i have not even mentioned. This is why I suggest swapping to a 97-02 drive train. you might be able to use the L26 and convert all the sensors to the ones from a L36. you would still need a 97-02 4t65e trans and 3.29 gears. Then you can run a 98/99 harness from the Z/ltz/police lumina that is plug and play, it will work with your instrument cluster and just connect right to your fire wall c100 connector. with 98/99 axles for the trans it is pretty much bolt up and go. if you find a down pipe from the 98/99 Z/asdfa that will be direct swap as well.

The bottom line is anything is possible. yes Im sure it is possible to get the 08 drive train to work in your 96. yes the parts are cheaper, but the time and fabrication and unknown issues that will pop up will more then end up costing more then just getting a l36 and 4t65e.

A lot of the info for the swap was out there... but over the years many of these forums have been shut down and the info was lost. there is a site called motorswap.org some info may exist over there.