Valve overlap in 350cid?

Let me think. That would be whichever cylinder is at the top of it's exhaust stroke. Based on the firing order, that should be cylinder 6 - the cylinder directly opposed to cyl 1 in the firing order. Similarly, 5 would overlap when 8 was firing, 7 would overlap while 4 was firing, 2 would overlap while 3 was firing, and so on.

This is true because each cylinder fires once every two rotations. In an 8-cylinder engine, that makes for 4 firings each rotation. Most engines (this included) are even-firing, which means that these events are evenly spaced. That makes for one firing every quarter turn (90 degrees) of the crankshaft. This means that the fourth cylinder to fire (6) must be at TDC at the same time as cylinder 1. However, they are at opposite points in their cycle. Cyl 1 has compressed and is about to fire, and cyl 6 has pushed out exhaust and the valves are overlapped, preparing for intake.

It has to do with the cylinder filling efficiency: by allowing some fresh mixture past the exhaust valve during overlap, it lets more fuel-air mixture into the compression chamber which means more power output from each power stroke. Improved cylinder filling means the same fuel quantity is used in a more efficient manner. This translates into better fuel economy. Imagine filling an empty, sealed vessel with water using a pipe at its side. At one point, the filling will slow down and eventually stop with some air trapped at the top, therefore it will be filled, say 95 percent. If a hole is made at the top, the trapped air blows out and empty vessel will be filled to the top (100 percent) in less time and with less effort (no counterpressure), with some water spilling over. That's what valve overlap is about.