When the SUU-23A was loaded on the centerline of the F-4 the short pylon was used. Do not use the long pylon. It was mounted directly to the centerline rack with the gun pod mounted to the pylon. The gun pod used the aft mounting lugs so the rear of the pod would clear the ground on aircraft rotation. The gun pod that comes with the Monogram/Revell kits are SUU-16A/A pods and are not very accurate. The Hasagawa weapons kit pods are very accurate and are SUU-23A gun pods.
Berny
Phormer Phantom Phixer
On the bench
TF-102A Delta Dagger, 32nd FIS, 54-1370, 1/48 scale. Monogram Pro Modeler with C&H conversion.
Revell F-4E Phantom II 33rd TFW, 58th TFS, 69-260, 1/32 scale.
Tamiya F-4D Phantom II, 13th TFS, 66-8711, 1/32 scale. F-4 Phantom Group Build.