360 engine sudden drop of oil pressure while cruising at 3200 rpm

About the only thing that could make an loud enough 'thump' sound is a pushrod bending/jumping of a lifter and the lifter jumps its bore as the cam knocks it out, creating a big internal oil leak.

I would pull the valvecovers first and check for a very loose rocker.
(The engine should've felt like it ran on 7 cylinders right after this had happened)