Will spindles from a 1973- 1987 Chevy truck work on a 1968 Chevy if you change the ball joints to the style that they used 1971 to 1972? Did they use the same still tie rod ends?
Use ball joints from the 73-87 trucks and yes it will work. (71-72 were transition years and I'm not sure they're the same.) The outer tie rod ends are huskier in the 73-87s, so I think you will need to change that. Your NAPA guy can tell you if the part numbers are the same for 68 and 73-87. Most of the major truck parts sellers can give you good advice on the tie rod changes that are required. On my 66 I had to use 73-87 outers and 66 inners (ES350L is part for that). To make it work you need a special tie rod sleeve with different threads... it's a common part. For details, try talking to classic parts america or www.performanceonline.com
