Detailed pictures might help. I would also find out the credentials of the mechanic or shop doing the work. When the engine went the first time everything should have been measured block,crank,bores for spec and the old parts inspected for problems as well (odd wear etc).
Having them tell you something is ok is not ok especially when a rebuild hand grenades after 50 hours. A good re builder should be able to measure and inspect every component inside your engine and easily see anything that will or could be a problem before if happens unless you had defectively manufactured parts. It's hard to diagnose anything over the net or by pictures.