how can i use a 25" motor on a 20" transom

[MoonGlow II]

Ok with all your help now I know I have a boat with a 20" transom Well it is just shy of 21" but. In looking for a used motor I am finding many more 25" shaft motors than 20" ones So what is the cheapest and or best way to use a 25" on my 20" boat...can I just raise it to the highest holes on the motor or will that still be to deep? Thanks Andy