Interesting thread.
A few thoughts:
About the turbo...I would skip it. I love turbos, but they concern me in an off the wall combinations like this that will be seeing more than just full throttle runs for short distances.
Tuning an aftermarket turbo setup on an aftermarket fuel injection setup is fine. People do it all the time to go fast. You can get good drivability and make sure the fueling is spot on in transition to full throttle and at full throttle.
Tuning for the high loads of towing at lower throttle openings, and when the boost builds because you hit a hill towing 9K, it entirely different. You will have overheating, detonation, fueling issues, overall it will be a huge pain.
If you have a GOOD and DEDICATED tuner that you are ready to pay, it is entirely doable. I'm just not sure its worth the gain. That 429 will make plenty of power to tow that rig with the right cam and gears. Also, you can easily swap the short block for a 460 and use all your other parts if you need a bit more torque.
The gear vendors is a fine idea, just for the overdrive. That will let you run more gear without spinning it to the moon.
As for the rest, my main concern would be the weight and wheelbase of the tow vehicle (as stated by others). I'm not one to freak out about exact tow weights and such, but that is a lot of trailer for a short truck, and it is 2-3 times your trucks curb weight. That will give the trailer a lot of control of the truck no matter how much brakes and power you have.