Thought you might like this pic.
On the right is a standard 1303 seni-trailing arm, in the centre is a steel 944 arm (which I think is identical bar the wheel hub assembly), while on the left is an ally 944 arm.
I was going to use the ally job on my '03 until I saw the extra width it'll add to the rear wheels!
Another point to note is that the bolt pattern for the ally arm will not fit the 1303 spring plate, where the steel will.
Hope this helps.