leveling garage floor

I am in the process of converting two single garages which are attached to the house into one large one.
The two are under the same roof and the wall in between is not supporting. Removing the wall will leave floor uneven and at slightly different levels. I do not want to reduce the head room at the garage door so what is the best option for leveling the garage floor? The existing floors are in good condition and relaying it completely would be a difficult option as the floor would have to be broken up before re-laying so ideally I am looking for something that will give a strong floor but fairly thin.
Can anyone offer a solution?
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