You sure its for 7/16 stud? all Hi-Po 289 were 3/8", have some 289 by 3/8" grooved lying around here somewhere, not sure of condition though.
I wonder if the rocker ball from the early 'base' 429 has been used in your application, as these were 7/16 stud with a 'rail' type rocker arm rather than the later pressed steel sled pedastal type. This may have been done to enable the use of the stiffer/stronger 7/16 studs in this application. Sorry, I dont have any of these here in order to cross check with original 3/8" type for compatibility with 289 rocker arm.