On small block chevies, there are 2 bolt holes on the passenger side front of the block just above the oil pan. I believe it's the upper one, but it passes through to the fuel pump pushrod tube. You can insert a long bolt in there to lock the fuel pump pushrod in place. When running, though, you must insert a short bolt to prevent oil from coming out. My truck did a very similar thing. If I were running under load with any kind of engine rpm, oil would leak out on to the headers and it would billow smoke. I could only get it to leak on the highway and it was like a mist so it left no trail. It was very difficult to find. I replaced the timing cover gasket twice and the oil pan gasket once trying to fix it before I found out about the missing bolt.