From my 94 LT1 TA. Was using as commuter car since 130 mile round trip to work every day and so used OD every day. Car had about 100k miles on it at the time and wasn't beat on a lot. One day going to work, shifted from 4th to 5th and nothing there. Tried 6th, nothing there either. 1-4 and reverse still worked and drove the car for a short while that way until I ran across a complete T56 setup from a 96 TA.
When it failed I was on freeway going about 60 and mildly shifted to find nothing there. There was no noise or resistance noticeable, it just has nothing there. So, my guess after some research makes me think it is just the 5-6 shift fork which appears to be serviceable by just removing shifter mechanism and tail housing.
Not sure if I should go through the whole thing while in it, but doesn't look any more difficult than the 4 or 5 top loader Ford trannies I have rebuilt in the past. I did pick up a kit of T56 special tools off EBay too, but it has sat on shelf several years while the tranny has sat under a Mustang gathering dust too.
See that keisler doesn't make their shifter relocation kits any more but a couple other companies do and since I want stock bench front seat, that would be necessary too. Not sure how to go about making the reverse lockout work in non-computer car, but figure there must be a way.. Not sure what to do with electronic speedometer if used it in the 60 and I don't want a non-stock appearing speedometer.
Dug out the tranny last night and cleaned it up and now sitting on makeshift bench. I did get a 64 3 speed with OD that will work in the Impala too, and also have a 64 T85 Ford OD tranny and adapter bellhousing of unknown origin that has Chev engine side and Ford trans side. Downside is GM 3 speed OD is light duty R10 OD and Ford HD T85 with R11 OD will probably cause some shift linkage and speedometer issues too.
I have this problem of keeping on changing the plan so short of a new flywheel and clutch, I have the two 3 speed OD setups with trannies and bellhousing said all cleaned up and ready to go. But having trouble deciding which to use so maybe not a great idea to add the 3rd option to the mix, but any of them should give me the OD which I cannot see going without. Otherwise I could just use a GM T10 or Saginaw 4 speed I have around here somewhere too, but see the 3 speed OD's as both preferable to those given where I usually drive (highway).
But being I have the whole T56 setup (pedal assembly, clutch slave and master, flywheel (that only works on one piece rear seal motors), clutch, PP and driveshaft (even the console), that should minimize the difficulty of getting clutch linkage and other parts to work. (Also have the LT1 starter as that is an issue using T56 as well). Too many options I guess. Gotta go, shop just called and said TA ready with AC pump replaced and check engine light issues hunted down and fixed (electronic and AC issues above my pay grade..)