Cannot seem to get Ubuntu Kickstart and preseed to cooperate with partitioning

I am trying to set up a minimal installation for some Ubuntu 12.04 workstations where I can pop in a CD-ROM, hit enter a couple times, and have it build me a ready-made system for imaging and deployment.

The systems have 3 drives, but since Ubuntu's automated installation methods only do a single drive, I am just trying to automatically partition the first drive. These machines have partitions defined already, the first partition is a utility partition I want to keep while the rest of the partitions on the drive should be blown away and used for the Ubuntu install.

To sum up, I want to:

1. Remove all existing Linux partitions on the first hard drive (the utility partition is vfat).
2. Create a single ext4 partition in that free space that is 159000 MB in size and use that as /.

Here is my problem -- Ubuntu Kickstart allows me the ability to remove all the existing Linux partitions but then instead of using the partition information specified, I get prompted for whether I want to use Guided options or Manual. Regardless of what I select at this point, the partitions it creates use the entire drive instead of being 159 GB. If I use Ubuntu partman via preseeding, it doesn't have an option for blowing away just the Linux partitions, it's all or nothing.

I've been trying to use a combined approach, where I specify a ks.cfg file with preseed commands in it, but I am still getting prompted for the 'Guided or Manual' options. Any ideas?

I am using the Alternate install CD for 12.04.2, amd64.

Here is my ks.cfg file:

#Generated by Kickstart Configurator
#platform=AMD64 or Intel EM64T

#System language
lang en_US
#Language modules to install
langsupport en_US
#System keyboard
keyboard us
#System mouse
mouse
#System timezone
timezone --utc America/New_York
preseed clock-setup/ntp boolean false
#Root password
rootpw --disabled
#Initial user
<snip>
#Reboot after installation
reboot
#Use text mode install
text
#Install OS instead of upgrade
install
#Use CDROM installation media
cdrom
#System bootloader configuration
bootloader --location=mbr 
#Clear the Master Boot Record
zerombr yes
#Partition clearing information
clearpart --linux --initlabel 
#Disk partitioning information
# Tried these two to fix the 'Guided' prompt but no dice.
# preseed partman-auto/init_automatically_partition select manual
# preseed partman-auto/disk string /dev/sda
part / --fstype ext4 --size 159948 --asprimary --ondisk sda
#System authorization infomation
auth  --useshadow  --enablemd5 
#Network information
network --bootproto=dhcp --device=eth0
#Firewall configuration
firewall --disabled 
#Do not configure the X Window System
skipx