You have some options . I have done the fulltime to partime swap on two different late seventies trucks, and kept the 4 1/2" bolt circle by using the expensive, and I'm sure by now, hard to find kit. You had to be careful of which partime 203 case kit to use. 205 is much better, and so is swapping outer knuckles to 5 1/2 " bolt circle that became standard after 79 on the 4x4's. I believe you could still use your pre 80 axle housing too. Not positive on that, but someone here must have done it.