How do I measure my spindles?

[7T2]

OK, on my 57 GMC one of the previous owners spliced in a modern front frame clip and front suspension. I don't know what year/make vehicle it came from. I have been told it looks like a 67-87 Chevy/GMC setup, and in fact I took a set of control arms off of a 78 Chevy truck an bolted them right up. I also used an air suspension mounting kit made for 67-87 GM trucks and it also fit perfect. The last thing I want to do to it is maybe add some drop spindles to it to get it down that extra couple inches. Obviously I would like to retain my current disc brake setup and not mess with any of that. I know there are different spindle sizes and diameters, so my question is: not knowing anything about the current setup what do I need to measure and exactly where do I measure it to find out what spindles I have, so I know what to replace them with? I assume I need to measure the diameter of the spindle itself, but where on the spindle is this measurement obtained? Also, are there differences in ball joints, and where do I measure those?