It's pretty straightforward and there is a lot of info out there on the web (check out Eberg MA article). The "73-76" A Body spindles are the exact same as the 73-74 Ebody. If the vendor says they are based on 73-up spindles I'd be surprised if this is not what they are offering. These in turn are the same as the 70-72 E Body spindles in all respects except with the larger inner bearing...which you want if for no other reason than to avoid the expensive and less reliable two piece early rotors.
There are two caliper brackets, slider and pin, each with merits, that will fit the same spindle and work fine with their respective calipers.

also available are the later F/M/J spindles that have been mentioned. They are a little taller but otherwise fit the same. Debate has raged for 20 years on these...IMHO they are fine to use.