There are a lot of factors that must be considered in a spindle swap. Ball joint stud diameter and taper is one, and it is a MAJOR safety consideration. The ball joint stud MUST match the spindle. It is possible to assemble mismatched parts and end up with an unsafe situation, so the fact that they bolt together is not enough. Mismatched ball joints and spindle can lead to a broken front suspension at speed. See the related thread on this site. Note that Speedway Motors sells tapered reamers that will allow you to machine the spindle holes to match the ball joints.
Spindle height is a concern. Changing the height changes the roll center. Sometimes this is a good thing (like the B-body spindle swap onto A-body cars) but sometimes it is not. You need to understand the impact.
Finally, steering arm location is an issue. The good news is that the 64-72 A-body spindles have bolt-on steering arms, so you could make or adapt new ones to work with your steering system.