I thought I'd just cheer somebody else (besides me) using a 6l80. If you have a local driveshaft shop, they can do anything you need, especially if you bring them sufficient evidence of what is there, along with specific measurements. Here's a cool link for measuring necessary length from the shop that made mine (to link the 6l80 to my C4 Corvette rear):
http://www.dlssac.com/php/trig2.php.