Steve,
The only safe way means partitioning the drive.
Didn't know the installer stopped you, but it is essential that the whole of the OS fits in the first 8GB if the disk. I put a 120GB disk in my old iMac G3, and I just had to partition it, even though I'm usually dead set against it.
If you have the need, like I did, you can have multiple partitions in the first 8GB, each sized for a particular OS. Can't remember the sizes I used, but I could fit OSX 10.2, OS9 and OS8.6 all in their own partitions, with plenty of space to spare in each, and another whopping 99GB partition for them all to use.
I'm sure some have managed to get the OS installed without partitioning, and as long as the OS remains in the first 8GB all will be fine, but all you need is an OS update, or some software install that installs some OS functionality to cause it all to go wrong.