Take one freshly cleaned gearbox casing
Add a gearset with new shinty baulkrings & big bearings
While mostly ignoring the frightening hints that a few big chunks of something had passed through the box in the past.
If you don't have any good way to hold the gearbox while tightening up the big nuts then improvise!
Then add a nice new diff
And a centre oil pickup pipe
Later after you throw the engine back on the top stick on a nice new timing gear set.