First, did you follow the tech section(s) here and at Rich's site(s) to set up the trem?
If so and you still can't get it level, more springs is your only option . . .
The spring retainer is a "safe guard" against the springs popping off, unless you're VERY violent with your trem, the odds that any springs will come off the block during use are slim.
I've used my trems both with and without and retainer and have never had a spring pop off, and I am a self-declaired bar abuser (I've broken not only the bar many times, I've broken a bridge.)
It CAN happen, but not as easily as you think.
Check out "All About EVO" on www.vai.com
, Steve doesn't use the retainer on his guitars and he's the King of "Vai-olent" trem abuse. :-)
Safe to say, use the screw holes for your extra springs, if you find them popping off you have two options . .
Use three, shorter springs or . . .
Try to find springs with hog-rings on both ends. They DO make them, I just have no idea where to find them.
Get two of these "double-ringed" springs and use the screws for the retainer to hold them, and the retainer, in place.