This is the time to get rid of closed drive shaft and it's differential. Get a good ratio, more modern diff, with cheaper, available parts. Could be WAY less work if you find a good donor. Could be WAY less money. Could be WAY farther ahead.
Switching out the rear end for a modern one sounds like good advice until things start to snowball & you run into more unexpected costs.
Pay for a donor rear end & then more than likely have to rebuild the brakes on it.
Will the new brakes be compatible with your existing brake system ?
{Example : Huck brakes on the front axle & Bendix brakes on the rear axle are a recipe for disaster}.
Then you will have to index your new axle in relation to the driveline & fabricate new mounts & weld those on.
Your existing transmission has a closed U joint set up which is not compatible with the new open U joint rear end.
You will now have to remove your original transmission & locate a new open driveshaft transmission & install that.
Next you will need to have a shop make you a driveshaft & balance it.
Then you will need to figure out a way to adapt your existing parking brake cables to the new rear end.
Most of the newer rear axles with better gearing are 5 lug.
Looks like you now will need to go to 5 lug rims, so you will need to locate 4 new rims & switch out the front brake drums to 5 lug.
After you have done all of this...you now have a used rear end that might last 20 years or it might last 20 miles
{ unless you have gone through the expense of rebuilding it before you installed it }.
As you can see....
The most economical & least intrusive way to change the gearing in your truck is to do so with your existing torque tube axle.
That is why the 3.55 gear conversion is so popular.
