A missing flywheel bolt will do the same thing, as I discovered shortly after buying my 59 3800 dually. It cost me a clutch in pretty short order, and while changing it, I discovered the missing bolt. The PO had cross-threaded the bolt during reassembly after an out-of-frame rebuild, and just discarded that one. I was able to re-tap the hole in the crank flange and replace the bolt, stopping the oil leak.
Jerry
