Randall,
I will have to look in the proper manual from the 50's to see exactly what fuze was appropriate with the HE projo, but it would have been PD, MT or Prox.
Typically the HE versions of shells are used as spotters for the Nuke projectile they want to fire. The 155mm and 8 inch Nuke projos are rocket assisted, and the M549 155mm and M650 8 inch were both rocket assisted to approximate the same trajectory. Since it is desireable to have an airburst for Nukes in most cases to minimize fallout, a MT or Prox fuze would be used. If you watch the Youtube video of the 280mm atomic gun, they show a MT fuze.