You may think you've considered these things, but.. you really haven't.
You're going to have to raise the bed floor at LEAST that much, probably more. And you'll need to drop the body that much as well in any attempt to retain handeling. So.. you've lost 12" (give or take) of your bed... that leaves you about 4" of bed between the wheels and forward, maybe a little more behind it. So your cargo capacity is going to be SUBSTANTIALLY diminished vs a stock C1500.
My point is your goal of retaining the benefits of both vehicles, is basically going to result in you retaining the benefits of neither. Short of building an entire carbon fiber bed, you will not be able to save 500 lbs. I would be willing to be there is not 500 lbs in the doors and the doghouse to save.. (even if you left them off completely). Now, say you do build a carbon fiber bed (read: $$$$$$)... you now have no weight over the drive tires (no, the transmission is NOT enough), so your handling has gone to complete ****. Bottom line, your end product is going to be a LOT heavier than a vette. So, your performance is going to be significantly diminished vs a vette.
Now, for lengthening the torque tube. That tube is made of Aluminum Metal Matrix Composite which requires a very specialized process to weld it with the proper strength. The only way to really do it, is to have the housing lengthened with a section of 5" 6061 alloy (read: very easy) and then have a carbon fiber shaft made (alloy won't handle one longer than stock). The factory length one offered by driveshaftshop is $1300... I don't even want to know what a custom length shaft would run.
At the end of the day, this is a cool idea, that will cost you at ABSOLUTE MINIMUM like... $20K? (Buy a corvette chassis for 7K, buy a truck for 2K, buy a driveshaft for 2K, 2K in a custom chassis build, 1K for a fuel system, 2K minimum for interior needs, 1K for gauges, 3K+ misc bullshit, etc etc...) That all assumes you have access to EVERY tool you'll need... and no matter what you have, you don't. And then.. you will have a truck, with very little cargo capacity, that doesn't handle, brake, or go anywhere near as well as the corvette, for double what it woulda cost you to buy corvette, and a truck. If I thought that I could be helpful in making this project successful, I'd be ALL about it. You need to realize that your cool idea, is never going to get off paper in a functional capacity.
Do what I did... go buy a truck, and buy a fun car... and spend your money modding them both to be good at what they are, don't try to make either of them what they're not. You will end up much happier in the end. I'll even take you for a beat run in the GTO to prove my point.