If you refer to JBL's W horn designed by D.B. Keele in 1977, note it was "designed specifically for the JBL K151 18-inch loudspeaker." This limits your choice of woofer. Keele used K 151 4 ohm for measurements as shown.

The E-151 (8 ohm) or E-155 (4 or 8 ohm) could possibly work also, assuming the parameters haven't changed too much.

The cab plans and explanations are available, i have these.