Been using NGK BP5HS plugs in my P6s for 40 odd years, never had an issue with them unlike Champion which must have had bad runs in production, at the garage I worked at we had cars coming back after a week or so with misfiring after servicing hence we used NGK from then on.
In my early silly day I did drive the granny out of the old V8 going right past the red lines on the rev counter until the lifters bled down at nearly 6000 RPM so my mates 3 litre Capri wouldn't over take !
Never had a engine let go or any drive gear problem.
I noticed that the bulkhead is pre cut for the factory air conditioning recycling inlet and centre cold vent outlet.
Seems only the earlier cars were done like this even though a /c was a factory option.
My 75 and 76 cars were not cut like that.