I've done it to my 32 gtst and it worked quite well. But now that i to am restoring while its stripped im installing a 33 rack and pump.
To route it through the cooler i just used a couple of barb fittings that i had from mounting a transmission cooler and some 3/8 oil hose. Was real simple to do and only took 15mins
Did you throw out an orange? No seriously when you were driving could you have hit a pothole or something that could cause your exhaust or whatever to hit the ground?
was the rubber boot around the gearstick installed correctly again? if its around the wrong way it has enough pressure to pull it out of 1st 3rd and 5th. Turn it around and see if its the cause
Just give your mechanic a call he should be able to do it, or even your local butcher .
IMHO i hate welded diffs, have never liked the way they drive and they're characteristics, but i also have a 2 way that sometimes acts like that