Salisbury diff cage hitting fill plug

Has anyone had this problem before? I went to change my diff oils tonight and noticed on my rear (salisbury) diff that the spider gear cage has been hitting the corner of the fill plug. Someone must have overdone the fill plug at some point. I put some ptfe tape round it to bring out out a bit.
I'm surprised the components are this close to the back plate. Or do you think I have the wrong taper plug in there?

looks like the right plug but threads in covers are weak and easy to wind plug too far through

Ah ok. Might replace the cover then. I was about the paint the axle. I'll wait till I replace it.