If it has a closed drive shaft, you probably want to change to open. That will be an opportunity to find a newer differential with 3.73 ratio. "Changing gears" in your diff or any diff can be very expensive and takes someone who knows what they are doing and has the tools. Don't go there for this truck. 350 motors, trans and rear ends are all over CA at reasonable prices. SoCal in one of the centers of the motor world. Plenty of help. Join a club. I really dig Panels and Suburbans. Should be fun to drive. Get the motor, trans and rear and put it together. Take off entire front clip and get a cherry picker. Drive shaft will be last. Make sure pinion angle is acceptable. Mock everything up, study it for clearances, fits, supports, radiator, exhaust, steering shaft and then bolt/weld/fasten down. Many here have been there, done that to your era.
