I'd suggest measuring the T/S parameters on your 416s before settling on boxes for them. Any two in one box need to be verrry close ITR. Impedance plots and matching that as well. AFAIK, the current kits are the one-size-fits-all 416C kit Altec was using when they quit, so if you recone any, you'll probably need to do them all for the best matchup, and even I would wince doing that to NOS Bs.

Another option to use what you have if they don't match well enough is a 4-box MTM.

Doing a passive XO for a 3-way with mid and HF horns of such vastly different dimensions... I wouldn't go there. Go active 3-way and dial in the XO and delay. Be aware though that not all active XOs offer delay for the HF.