The 87840243 is a NH number, so that would be new enough to use the 9/16" bolts.
I'm sure the casting numbers on the head changed, but I do not have that specific info. That's really getting into the weeds.
I would certainly consider drilling out the holes if it were mine. You wouldn't have to remove much to get the 9/16" bolts to pass through because the existing holes are already quite a bit bigger than 1/2".