I have a T56 in my 90 sportside which originally had a 5 speed and yes the driveshaft had to be shortened and a couple other minor alterations, but it was well worth it. You can use the early T56 transmission with the 2.66 first gear against the 6.0 but you must change the input shaft for the hookup and while you're into it, changing the syncros from aluminum to steel billet and install bronze bushings is a hot ticket. On another note you really do not have to use the rear shifter position as there are aftermarket shifters to reposition the shifter to the upfront position. In my case I wanted to use my stock bench seat, so I had a C section cut out of the seat approximately 4 inches back and 4 inches across by a friend who is an upholsterer and the shifter lays right next to you. I've never taken photos though I should but this was the first of the mods on my truck and although I have a fresh 96 LT1 for it, I too am considering a 6.0 but that may never happen. If you do go this route using the T56, you will have to run at least a 3:73 rear for drivability, otherwise you'll be feathering the clutch with the 3:42's as I believe that's the gear ratio all 5 speeds came with.
Hope this helps