You would have to do lots of work to the tank to get it to fit the '89 frame. It's not just the brackets that are different.
This is what the '89 frame looks like: (actually a 500 frame, but very very similar to the '89 250)

Note that it's got the older style single backbone under the tank.
This is a tank that fits this type of frame, note the recess in the bottom that fits over the backbone in the frame:

Now, this is a newer style perimeter frame. Note that there is no backbone in the frame, but the two "rails" that run outside the tank:

...and this is a tank to fit this type of frame:

You can see here how the center of the tank dips down between those rails on the perimeter frame, where the backbone is on the older frames:

So it's much more than just the brackets that are different. Neilios said his tank is a one-off tank, with modified brackets on the frame. A standard run-of-the-mill tank for an '03 just isn't going to fit the '89 frame.
Don't take this the wrong way, I mean no disrespect, constructive criticism at worst. Think if it as helpful advise, and take it as just that. Don't take my word for it, but do check it out for yourself before you spend the money on a tank that isn't going to fit your bike.
