Welcome to the 'bolt 47Chev , ,I'm sorry you have had a bad experience with your repairs .
I have done some of these , and all the okie bushings have come from Jim Carters ,, is this where you got yours from ?
One I did , the bushing was 0.008" to large in dia to go in the torque tube and I had to machine it down to the correct size ,,
But I have just done one on a '37 that was perfect in every respect .
I think that perhaps the quality control could be improved there .
On the other part of your post ,, the installation of a later diff is not too difficult , mostly making the decision of what to choose can be confusing .
I have a '55 truck diff in my '40 pickup, it is an easy fit and has several benifits for me .
If you really want to go this route I will expand further on your request .
Give us more information on your troubles and what you hope to achieve next .
