My god....any garage that has concerns about changing the rear shocks on a 75 should be shut down !!! It's only two bolts and four nuts - the bolts can be very tight, but a breaker bar and plenty of Weetabix will see them sorted
Not too sure how they can mess up a water leak though ?
But no one owner, 42k car needs to be deemed dead....get onto Mickeyboy as others have said, I've bought spares from him before and his prices & service were excellent.
Good luck with sorting it and hope you can find a decent garage soon !