I got a couple 300Ds recently for cheap (a complete rusty '78 and an engineless '83 240D with a good body/paint).  I'm trying to get the '78 running - It had a torn-up flexplate, and the starter wouldn't engage.  I hear these flexplates have an external balance, but the old one is torn up, and the new used one seems to have a different balance.  Unfortunately, I removed the old flexplate before realizing this.

I've heard that some folks have just R&R'd these things with no trouble.  Anyone had a different experience?  If the thing goes back together and vibrates like crazy, how on earth can the new flexplate be balanced to the engine?  If this is not the right forum, can someone point me to a more relevant one?  Thanks from an old gearhead...