You will have to change the rear diff to match the transmission, or the engine will try to spin the front and rear wheels with different speeds.
At 70+ mph you're at what, 3500rpm? That's not that bad for the engine... most of the people here with 98-01 RS's (which have even shorter gearing) have well over 100k on their cars with plenty of 70+ mph driving. I wouldn't worry about it... a new transmission/rear diff will cost you as much as a new engine, so it is definitely not worth the trouble.
At 70+ mph you're at what, 3500rpm? That's not that bad for the engine... most of the people here with 98-01 RS's (which have even shorter gearing) have well over 100k on their cars with plenty of 70+ mph driving. I wouldn't worry about it... a new transmission/rear diff will cost you as much as a new engine, so it is definitely not worth the trouble.
