Well, the plot thickens. After perusing 42 years of old receipts it turns out that the Midas Muffler Shop my brother took the car to, replaced the driver's side spindle about 30 years ago when they did a brake job....and they used a 1978 spindle, and didn't bother to fully tighten the ball joint castle nut. Hence the "D58" date code on the spindle and the noticeably lighter duty casting than the passenger side spindle which is a 329354 spindle dated "G25". So I guess I need a 329353 spindle in case anyone has one sitting on a coffee table somewhere. I believe that this spindle was used on just about every disc brake GM car from 1970 to 1976.
Driver's side 1978 spindle:
Original 1975 passenger spindle:
