So ( just my 2 cents ) my suggestion is to
put off for a while
the building of new cabinets.
In the meantime use what you have there to figure out the following ( before moving on + building your new ones ) ;
(i) Optimal size ( determine best size for your circumstances and the realities of working with Altec 414's )
- One can always reduce a boxes net volume by temporarily placing known fillers into said box and then adjust box tuning to experience the effect of the smaller box.
(ii) Optimal box tuning ( determine the best box tuning for your room and musical tastes )
- You'll need some box tuning software and the ability to actually measure the resultant box tuning via impedance curves > but this is well worth the effort to learn > since it'll result in a custom tuning for your room .
(iii) Optimal internal damping
- ( ie; determine whether Altec correct in it's minimalist approach of just lining 3 of 6 internal surfaces ? )
(iv) Optimal internal bracing
- ( ie; see if historically if Altec correct in its minimalist bracing approach > typically using diagonal bracing > according to their own enclosure building guides )
EDIT:(v) The best vertical arrangement ( for those twin 414's )
- Unfortunately, only making a test baffle board can accomplish this task / which means cutting out the older baffle board on at least one of the existing speakers > something that may be un-palatable.