Yeah, you'll need a driveshaft cut to length and balanced.
But, the C6 shouldn't have any issues with clearances in the 67/68 unibody.
However, mock it up and install it just to be sure. You can do this relatively easily with an empty C6 case. Get a junker, pull the front pump and valve body, then shake the guts out of it and *voila!* lightweight mock up trans case. Bolts to the bellhousing and your mount, lets you see what's up before you get the real one in there, and is easier to wrestle around even if you're using a lift. Costs maybe 25 bucks for a cracked case core or a junkyard part is a bit more, but not much. Not much demand for the c6 nowadays in the used parts biz.