That's a hot plug. It will add to your detonation problem.

The last digit specifies where the plug is in its heat range. XR4 is a plug that cross references to a JY12 to JY14. This plug would be on the hot side for a 7.5:1 motor with a 2.76 gear in a 5000 lb Imperial that almost never sees more that 2000 rpm.

I would not put anything hotter than a -6, like a NGK r5670-6, and would consider a -7.

And I also agree with what folks are saying about your full advance timing (>50°) being too high, and possibly being too lean. Plugs is the easy first step.