a 518 is basicly a 727 with a OD tailshaft. look at the rebuild kits/parts available, many will be for BOTH the 727 AND the 518. There are plenty of HD parts available.
From what I've read/been told the OD unit failures are a poor maintenance or misuse by owner issue. I'm building a 518 46RH (lock-up Tq) to go behind a 450 hp 440 and it will still be cheaper than a gear vendors, including the cost of the new bell housing.