Try spinning the oil pump with the line disconnected at the back of the pushrod chamber. You should get a good flow of oil out of the block. If not, the passage to the cam bearing is clogged, and you would probably need a complete teardown to a bare block to clear it. Sometimes putting high pressure air into that port at the back of the block will clear the clog, but I don't like to do halfway measures. The external line works, and it's been done that way for 50+ years.
Jerry
