Your TA would have come with the 489 case with the cone style sure grip. I would get a new or used power loc (clutch type) and correct gear ratio for your car (could only be 3.55 or 3.91 and only listed on the BS) and then use the 489 case you already have. If the 489 case you have is '72 or '73-up, it probably won't be machined for 3.91 gears if that is what you need. But, you can grind the clearance needed with a little effort.