Pardon me for jumping on this a bit late, but I've also got an Antec 300, and I'm using a 3.5" HDD (just like most people)
First off, you mentioned that you aren't using the larger philips screws (the ones with the big fat ends that you can tighten by hand) to secure the motherboard to the spacers. I chose to use these because you don't need to be fiddling around near to your motherboard with a screwdriver. This isn't really a problem since it's turned off, so you won't damage anything either way, unless you're really clumsy that is.
Also keep in mind that it would be a horrible design practice to put sensitive electronics near to the mounting holes on a motherboard. All motherboards should have a small amount of clearance around mounting holes such that there is virtually no risk of "shorting".
There is always the "beware of static electric shock" or something like that on the bag that wraps the hdd, so I assume even the hdd case is prone to shock.. If it's just a Faraday cage, then it doesn't matter if the hdd's left out in the middle of an electric thunder storm?
There is very little, if any risk to the HDDs in an Antec 300 from ESD, despite the fact that the metal sides of the HDDs are in contact with the metal case. Keep in mind that if your PSU is plugged in, the whole thing is grounded.
I've seen better hard drive mounting systems, but I've yet to be convinced that they are worth it, unless you plan to move your computer around a lot, in which case it might be better to get a different case or use a 3.5"-5.25" adapter with rubber padding, or similar.
Finally, the sensitive electronics in a HDD are isolated from the outer casing. If you refrain from touching the PCB (if not grounded), secure the hard drive well and connect it properly (power & sata), then you shouldn't have any problems.