Is the '78 4 wheel drive? If so it could add some cost of repair down the road like bearings, hubs, drive shaft u joints and possibly front axles or u-joints for the axles or even a transfer case. The 3/4 ton truck is a gas hog and if I didn't need that much truck would go with the 1/2 ton.
You can rebuild either of those trucks with basic and very inexpensive parts or you can update em with fairly new engines / transmissions if you want. If the 78 is 4 wheel drive check everything real good, make sure the front is in good shape (if not I'm not saying pass, just take it out of the buying price if possible) and you'll have a truck that can be a daily driver for many years
I love the old Chevy 4 wheel drives too. I'll buy a project truck and rebuild it one day.