On my 8.8 and 9.75 the passenger shock mount on the axle can go either way. does your mount on the axle have two holes? Take a pic of it.

 

One thing that was overlooked is the driveshaft. The plane where the 9.75 differential pinion flange bolts to the driveshaft is about 1 ½ inch closer to the front of the truck. Your driveshaft will need to be shortened.

Take a closer look at the lightning rear axle. It might be easier to weld a flange on the tube to mount the shock backwards like your 8.8.

 

the mount on the passenger side of the lightning rear looks exactly like the driver side mount but welded to the axle tube facing the opposite direction and on the other side of the rear. there is literally no way to mount the shock facing the rear with a style mount.

I tried to take a pic, but the flash wasn't working and it wasn't coming out. I will try again in the daytime.

I was told by another guy that's what I'm 9.75 rear (not for a L though) got to stop drive shaft should say no problem. I will go ahead and do the swap whenever I find the correct body shock mount, and if I run into the driveshaft problem I will deal with it then. One problem at a time.

I don't want to cut and reweld the mount facing the opposite direction because the single forward facing shock is much better for handeling. that would be a lot of work for a down grade.

 

My truck started life with a 4.6 so it had the 8.8 rear end. When I swapped in the 9.75 I had to shorten the shaft. As long as you keep the 4.2 stock your 8.8 should last. Why not just fab up an upper mount for the shock. If the holes are there..... Hell I wouldn't be surprised if they used the same mount and just moved it to the front.

 

On my truck I have both forward and rear mounting locations on the frame and axle on both a 8.8 and 9.75. Make a custom mount. It only has to hold the resistance of the shock and should be strait forward if you know or have a friend that welds. I guess I was lucky in the mounting dept. I had to switch to a 9.75 because I have grenaded 3 carriers and gear sets. 3.55,4.10, and 3.90's. I now have a 9.75 code L55 and has held up. Good luck on your swap!