Still watching this with interest. I would think that building the woofer boxes as modular units (somewhat as to your original design) would be the most efficient process. Adjusting box volume and port tuning could be done to each woofer, as needed. The boxes could be stacked for evaluation and then a panel could be removed from one box to combine the two, if necesary.

Locating the tweeter could be done after the boxes are built, trying different vertical positions in order to present the best dispersion and results to the listening location. A permanent mount would follow. My gut feeling is that a top-most location (above the mid horn) is going to bring the best results but your efforts may prove different. As to difficulty of mounting the tweeter above the mid horn, a good "cobbler" could have a bracket assembly built for that in short order.

I've been following other "three-way" Altec builds and I think there is a lot of work to make the tweeter integrate well with the rest. I am looking for a better active crossover to help with that. I initially used a Behringer CX3400 and it sold me on the use of active crossovers, especially with Altec Model 19 drivers.

Good luck with your project, looking forward to your results.