Build.prop is for the x900 itself,but it's clearly ICS
The image is part of the build.prop
It now seems to be a ICS build by seeing the bild.prop you attached. But someone who is as good as Ricky, or any other alike, could try it out and guide us then it would be helpful. Coz i don't think anyone here wants to risk bricking their device and not knowing how to get it back in working condition. I personally don't wanna risk it. The tool might be very legitimate but it still requires skill. This is my first android phone and I have never flashed Custom ROMs. If this tool really works, it'll be the first for me.
As for the update.zip of 348MB i think it's the ROM after the 59MB update.
Also, I had a look at the tool, it does have some encryption files which is why i'm optimistic that this tool signs the ROM. Also, users facing problems of rooting after the 59MB update can be solved as the tool also allows to push apk and other files in the ROM after flashing the device. This module of the tool can be used to push files required for root.
When you load the files into the tool kit before flashing it asks for a xml file. The xml file with the update.zip contains the instructions as to how to flash the device. But since it boot-loader locked it wont just flash any ROM. This is where the .bin fiiles come in to sign the ROM before flashing. This is what I think. I may be wrong though. I'm no advanced user.