..to get the 1/2 ton I really want...
The question I have is shortening it by 21" insane and will I be able to get a drive shaft that I can use that is not a 2 piece that will work in the stock transmission.
I still say this is a lot of work and not the best idea. If you want a 1/2 ton buy a 1/2 ton, there are lots of them out there.
Just shortening the 1 ton frame may not solve all your problems. What about axles and springs? do you want 1 ton stuff or 1/2 ton stuff? There may be other small issues that crop up.
It may still be simplest, quickest and or cheapest to buy a good 1/2 ton or just a parts truck, or pickup a cheap or free frame at the least. I know they are out there, I looked about a year ago and found several that were very reasonable, one free (didn't purchase any

better finish the first project before starting a second..)
No, shortening the frame 21" is not insane, hopefully you can make the crossmembers work out in the process, and you do an excellent job prepping and welding, or have it done professionally.
You can have most any driveshaft you could ever need modified or made new, really not all that expensive. I expect going to a one piece shaft will be OK. Here's a Tech Tip on drivelines
http://www.stovebolt.com/techtips/driveline/setup/index.htmlGrigg