Wow, what a great wife you have! Not too often will a wife tell the husband to go spend more money...
All joking aside, you might want to consider the 6L80 over the 6L90 if you're going the 6-speed route. They're basically the same, but the 6L90 is a little heavier duty and only comes with (as of about a year ago) either a 1410 or 1450 u-joint in slip yoke form. The trans slip yoke on my 6L90 is a 1410 and has a "conversion" u-joint on it to mate a 1410 to a 1350 at the trans end. If you aren't familiar with it, a 1310 was standard on these trucks (or at least my C10). 1350s are a typical upgrade for a performance build. 1400 series u-joints are typically for 1 ton and above vehicles. Or so I'm told.
When I went to order my driveshaft, I had a few places tell me they don't have the equipment to work on the 6 speed transmissions. I found one in San Antonio, Texas (not to far from me) that was able to build me a 3.5" aluminum shaft for this. Check them out:
http://www.driveshaftspecialist.com/ I'm sure there are others that can do it, and if you end up with the CV joint type mount, this might not be an issue at all.