If you use cushions to get your eyes a couple of cm higher make sure they are nice and firm. You dont whant to sink down when the G kicks in. Even minimal G will push you back down.
If your a/c does not have any good horisontal line like top of the dash doard then you might consider putting a piece of tape on the windshield. Once a piece of bubble gum did the trick... Some adhisives on tapes may damage the winwshield, so be careful.