So I know that the 67 axle is wider than a 64, also the axle housing is shaped differently. but what would cause a person to want a 67 axle where a 64 axle should go?

 

Well if you wanted late model wheels...you wouldn't need spacers because the backspacing would be a lot closer to working

 

that helps.

but this car was last on the road +20 years ago. what would of been the advantage then??

 

Cheap.

Is it a 'T' code car?
Is it a 9 inch housing?

Does it have a 3.25 gear set?

There are quite a few very viable reasons for it.

If it is a T code...it's because the stock 7.5 wasn't good enough and the later housing was available cheap at a pick and pull
If it is a 9 inch housing that is self explainatory
If it has a better gear set than 2.79 in it, it again might have been a cheaper alternative than a new pumpkin

It's easy enough to have it trimmed down if you want an exact fit for the car.

 

WES-T1 8JD
2.75 9 951A

yeah it's a 9 in but it's got 2.75's in it. talk about fun sucker.