If you are using the HU, and basically zero control (TA, power, cut off/xover, freq adjustments) then just buy what you want and stick em where you have space. If it was me, id try and get tweets higher, power them, cut them off proper and band tweak them. That said, if you can fit something larger in that space, like a 5-1/4 with a tweet or something, then id do that. Other wise, just buy something and put it in. Cheers.