I started a thread a while back about the changes made to each year. This mostly covers the cosmetic changes to the outside. Looks like I need to go back an update it with better info on the internals. You can find it here:
http://forums.luvtruck.com/viewtopic.ph ... es+changes
On the clutch, they were all hydro up until 78 when they went to cable. I've had both cable and hydro clutch trucks. I adjusted the cable on my 79 when I first got it, and again when I replaced the clutch. (spring flew out of the disc and lodged in the pressure plate. Maybe 5 minutes of work each time. The 76 and 77 trucks I've driven, I put a new clutch master cylinder in each, rebuilt the master cylinder on the 77 again once after that, rebuilt the slave cylinder on the 76 twice, replaced the slave once on the 77 and rebuilt it three times. Every time I worked on it, the pedal and slave both needed some adjustments. I've probably put more time into the clutch system on the 77 than I have the rest of the truck. Given a choice, I'd pick cable every time.
The disc, pressure plate, and flywheel may be different between pre 76 trucks and post, I honestly never had to look. There is no reason that the change in how the pedal operates would have forced any of the rest of it to change, but I dimly recall something being different. Last I checked, the full kit was going for about $80 on ebay, also check
http://www.rockauto.com and see what a full kit costs. Given the amount of trouble it takes to change out the clutch (pulling the trans, etc) I would never put any used parts back in except the flywheel and that I would probably resurface if it looked sketchy. The kit comes with all the parts plus an alignment tool, making it a pretty easy install.