I too would keep the horn separate & up on top.
Doing so allows adjustment of the "Z" axis ensuring proper summation in the crossover region ( so says the theory > in practice one needs measurement equipment of some type to check the FR ) .
Personally I prefer the look of woofers that are counter-sunk and mounted to the front of the baffle ( that's just my own bias mind-you ).
OTOH, you might not have any choice in how you a-fix those older 414's ( I don't know if they can be mounted any other way but to the back of the baffle board ).