Depends on who you ask!
Rick Ehrenberg will tell you that using an FMJ knuckle on an A-body will cause "ball joint overangling" and poor suspension geometry. Of course, he never actually checked to see if any of that actually happens (it doesn't).
However, if you actually plot it out like they did here
Swapping Disc-Brake Spindles - Mopar Muscle Magazine, then what you end up with using the FMJ spindle is improved camber gain, a very slightly better roll center, and a very small increase in toe change (bump steer). So, if you run wide tires which are more resistant to toe change, you'll likely be even better off with the FMJ's than A-body spindles.
I would say that the other advantage to the FMJ spindles is being more likely to find them at the local wrecking yard than an A-body spindle, but neither is very likely anymore. Although based on how the reproduction spindles are being advertised, I would imagine that they're based on FMJ spindles, not A-body spindles. Which would mean the only way to truly get a 73-76 A-body spindle would be used.
