Did you fit a new oil pump when you rebuilt the engine? The most common failure on original 253 and 308 was the cam, and the problem was created by the oil pump. You should buy a new pump each time you fit a new cam to be certain. It sounds to me like you have a r00ted cam. Pull the valley off and pull a few lifrers, you'll know pretty quickly if the cam is shot. Fit a new cam, lifters, oil pump and get as much old oil out as you can. Put new oil in an run the cam in to manufacturers requirements and fit a magnetic sump plug.