In search of a Y throat adapter

Hi,

currently I'm converting a 770 transistor Leslie cabinet into a tube powered 160 W one. As I didn't find any mid-range compression driver that could cope with this power, I decided to install two Monacor KU-516's. These drivers feature a 1 3/8" thread. Are there any adapters that allow connecting a pair of them to the rotating Leslie horn? Four bolt flange mounting to the spindle plate would be preferred.

Best regards!