a hard disk with pre installed snow leopard(10.6.3+)
[NOTE:The Hard disk should have atleast 2partitions(can have more), in which one partition should be empty for installing mountain lion and they should be GUID partition table formatted] how formatting using GUID?
(should be done during snow leopard installation)
mount the downloaded iso/dmg
open terminal and typedefaults write com.apple.Finder AppleShowAllFiles TRUE
now type killall Finder
now open the mounted disk
navigate to "system/installation/packages/"
install the same way as you did, select the empty drive made for mountain lion(can be named anything)
do not select the bootloader and bootloader options
these options will already be selected, let them be
just click sleep enabler
in the mac model select, MAC Pro 5,1
in drivers do not select sound(it usally gives problems in amd)
select the other drivers if you need, for example USB3, etc
do not select any driver if you are not sure, this can be done anytime when in installation is done(especially VGA)