GE PowerMark Gold + Conversion Kit
You need to use a backfed main with a THQLRK3 hold-down...
Your panel, unfortunately, is not compatible with the TQMH000 main breaker adapter kit you are contemplating.  This is denoted on the diagram by the box labeled &quot;MAIN&quot; taking up two branch breaker spaces, instead of being above the branch breaker &quot;stack&quot;.
...or just get a larger panel
However, there's an easy way to forestall all this, and that's simply to take the panel back and get something chunkier.  Going up to a 24-space, 125A or even a 30- or 40-space panel isn't that much more expensive, and means you don't have to agonize over panel spaces nearly as much.  You won't need to replace the main breaker in the bigger panel, either, since you're fitting a subpanel.  (It's either an easy way to get the required shutoff means for a subpanel in an outbuilding, or can simply be replaced by main lugs if the subpanel is in the same building as the panel feeding it.)