I dont have a whole ton more pics jsut a lot of explaining to do since i have a 3900 just the same as the g6 did i reused the intermediate shaft on the trans and then used cv axles from a 2017 gmc acadia front right and 2013 impala front left which splined in perfectly and don't seem to cause any issues in regards to axles, the shifter has a wiring harness go right behind it so I had to take and make a bracket over that harness and raise the rear end of the shifter up it definitely isn't perfect but it's solid and fits i also ended up putting in a homemade short throw shifter too by cutting and repositioning parts of the shifter rod I get somewhere between 30-50% shorter throw

 

I admit, I am a bit ignorant on the G6. Did it come with the same 3.9/3900 v6 as your Monte?!?! If so, I bet that actually helped quite a bit in this process! That explains your ability to really work off the G6 tuning file. Curious what it takes to address the issue to make the car believe it is in "park".
Been a long while since I have seen something new in and custom to a w-body (the group of people who tinker with these is shrinking).

 

I agree yes the 06 g6 gtp and 07 g6 gt specifically with a special sports performance package came with the same 3900 and an f40 trans they also came without that special package with the 3900 and 4t65e combo and that did help a lot because I used a factory g6 clutch and flywheel resurfaced and all factory parts to install everything minus a cavalier master cylinder which is a lot easier to make a mount for on the firewall as it only has 2 bolts and the hole for the boot instead of a 4 tab quarter turn interlock style and my idea to trick it into thinking it's in park is taking the neutral safety switch and the auto trans harness and figure out what wires need to be shorted to trick the computer then make a tiny harness with the clutch position sensor and the parking brake to trick it into park and neutral respectively

 

Some place on here there was another MC owner that changed his out to a Manual Trans. but it has been a few years ago, I think he did it to a 5th gen carlo. I will do some searching for that.
Great job! Fun stuff to do. .

 

AH! That sounds like something that should work.

Something with a 5th gen sounds familiar. Not sure if it was a manual trans or a LS4 swap....