Re: need advice - 1/2"" sheetrock replacing 5/8"
Why are you required to use 1/2" where 5/8" exists? The shims and man hours required to jury rig 1/2" will far exceed the cost of a sheet of 5/8" drywall.
In lieu of sanity, the shims that dj1 suggests would be the way to go. Keep in mind, if you're hanging this in a ceiling/overhead situation, the drywall is going to start sagging in short order because it it isn't strong enough to resist it. 5/8" is made for such applications. Also important to note, if you need to maintain a certain burn rating, 1/2" isn't going to come near what 5/8" will. Make sure that your "requirement" isn't going to become a safety hazard.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!