My 3/4 ton has a short closed front section of the driveshaft to a carrier bearing, back of that is open drive. The cast iron powerglide would not be worth it and all the ones I've had were FREE because they are not worth a dime. Modern overdrive automatic would be wonderful in your truck, just need to purchase an adaptor. Check out adaptors at Langdon's Stovebolt site. Also what ever you go with set it up with a carrier bearing as original to be safe. A friend of mine totaled his 3/4 ton truck after installing a long one piece driveshaft which came apart and pole vaulted his truck off the road.
