If you want to venture beyond merely replicating the existing design, let me advise, unequivocally, that you cannot do this successfully without measuring as you go.
Now, that doesn't mean a big investment in gear, if you can use your computer to do it. You'll need a software package like TrueRTA (1/6 octave version,) a mic preamp, the Behringer measurement microphone ($50) and a mini-boom stand for it, and an XLR mic cable. I haven't tried that package myself, but several others here are going (have gone) that route. Perhaps they or others will be able to advise further.
For starters, build yourself a LF box. I don't know if M19 is the optimum size and tuning for that woofer, but I'll plug it into BB6P for a preliminary look. As you know, I don't do Altec, typically. Perhaps others will have some suggestions on that part.
Once that is complete, the real fun begins. You'll be able to see how various driver/horn/network combinations work and sound in comparison to your M19s, and you'll be able to dial THOSE spot on to your listening space and frequency response preference.
O.K., what's the specific Altec woofer model number you want me to run in BB6P then?