A-body vs fmj spindle for handling?

Well i don't believe that.
That "editor" doesn't know what he talks about in lots of instances.
Weigh them yourself and see, if you have both styles, in your inventory of parts.

Great idea! Thanks for your help!
I’m not home for the next few days but I will weigh them when I get home. In the meantime, here is a link to the original article in hot rod magazine. Picture 3 of 10 explains the 3lb difference and you can clearly see the contoured back and extra material that is removed compared to the a-body spindle. Bill Reilly from Reilly Motorsport is the author.

Swapping Disc-Brake Spindles - Mopar Muscle Magazine