Yes, the 94 hard top will fit directly onto an 89 using the two L brackets that mount directly behind the door pillars. The hard top became an option, CC2, in 1989. To use the hard top on previous years, a GM adapter kit was needed. I have only seen one adapter kit available on the internet in 5 years. You will like having a hard top, but it is a two person job getting it on. A one person job if you have a pulley system for getting it off. You must have the brackets to get the roll bar effect. I would not advise buying the hard top if you don't get the brackets as they are no longer available. The bolts that attach the hard top to the bracket and frame costs about $7 at Chevy and they are still available. You should get both wrenches with the hard top. A torx that is used to attach the hard top to the windshield area and a closed end rachet wrench 7 mm / 8 mm. You can get the latter at Sears.
Barrett