help needed

If the 71 has an 8.75 rear end you should definitely keep that. It's much better than the one in the 75. If you need to change it to the larger bolt pattern you could just get new axles for it, or have the ones in it drilled for the larger bolt pattern.

On the front suspension and steering I would recommend just taking everything off of the 75. It should all bolt right up as long as you keep all the parts from the same year.