if you really want to keep the 944 arms you might be able to modify a aftermarket rear drum, a nice one with extra material for odd bolt patterns, i think CSP has them (this the one you got?) and fit the stock '03 gti brake to it sort of like how Al B did it here
http://www.germanlook.com/Forums/showthread.php?t=5674
but why not use beetle arms, and 944 brakes with spacer? this may be cheaper and easier, get lanner to make one thats about 1" thick with the two bolt patterns and the hub centric boss. sort of like this