Hope you never used a one oil hole head gasket and then installed it backwards if that's possible. I believe oil for the heads comes from cam bearings through a gallery in the block , through the head gasket into the head and into the rocker back bone . I believe the oil comes up through the second from front dr. side rocker boss and the second from the back on the pass side . Its been 25 yrs since I built a 340 so I could be all messed up It seems to me its either head gaskets on wrong or rocker back bone on up side down .