Thanks @Rusty, this is a big help
I loosened the rocker arm, swiveled it out of the way, and pulled out a rather bent pushrod
Dashed off to the junkyard and grabbed a handful of pushrods from an '82 305.
Used a washer and a socket to tap the split keeper back into place. Loosened up the valve a little in the process too. Sticky valve has not been cleared as a suspect at this point...
Popped in one of the used push rods, took the slack out of the rocker +3/4 turn for lash, and fired her up. Sure enough, sound gone!
Added some MMO to help free up anything that may be binding and causing trouble. If it happens again I'm ready