ported boxes

There's no such thing.  Port through the back if you wish with no changes to port formulas.   The phase shifts at the Fb regardless of port direction and there is inherently a greater group delay centered at the box freq with any vented design;  doesn't matter whether it fires forward or from rear.  What you hear at your listening location is the effect of the total sound, which includes interaction with the vehicle itself.

Your concern in this design should be about how well this interaction of driver / port / vehicle sounds to your listening position.  It may even sound better than a front-firing vent...you'd never know unless you tried it both ways.  And since that is always the case, build the enclosure so that you allow maximum amount of airspace per recommendation (or model) and shape it so that it fits the vehicle in the best way for your needs.  Allow several inches of space at the port end outside (as well as inside) the box.