Electro-Voice large-format (3" voice-coil) drivers are now sold with a 1.4" throat and require an adapter to fit their HP horns. The adapter is the ADH-6. You may be able to drill this adapter for the Altec driver.

BTW:
Years ago EV marketing had a built-in 1.4"-to-2" adapter on the throat of their large-format drivers (DH1A for example). This was EV's way of competing with the 2" driver craze generated by JBL marketing. All large HP horns have "vanes" in the throat to reduce standing waves due to the 2" throat size. The HP version sold by Altec did not have the vanes. It seems the 1.4" throat did have a standing wave issue . . .