Joey,
I have no experience as a security guard, but for what it is worth, I think you are on the right track with the Glock in 45ACP. I would probably opt for the 40 since you get the same stopping power and a couple more rounds in the mag. I would carry plenty of spare hi-cap mags. Almost all amoured car robberies I have ever heard of were by multiple assailants, and usually well armed. Personally, I think they should issue the best vests and kevlar helmets to be used outside the truck since so many guards are shot from behind while they are distracted. The helmet and vest together would almost insure that you would not suffer a fatal wound from behind and might just be enough to make a robber think twice about trying to shoot you from behind because he would know that as soon as he fired a shot, you (and your partner hopefully) would return fire. I just think the helemts would be a very practical safety feature and psychological deterant. I know companies would not like the PR and guards might think the helmets were uncomfortable, but helmets would save lives and possibly loads, so the companies should be all for it.
I hope this wasn't too far off topic or too much unsolicited info.
Roll Tide