Where are the exhaust valves in 350?

The location of the exhaust valves in a 350 engine depends on the type of engine. "350" refers to a displacement (350 cubic inches), not a specific engine design. There are many different 350 cubic inch engines, primarily small-block Chevrolet V8s, but also others from different manufacturers.

However, assuming you're talking about a small-block Chevrolet 350 V8, the exhaust valves are located in the cylinder heads, specifically on the exhaust side of each cylinder. Since it's a V8, the heads are arranged in a "V" shape, and the exhaust valves are generally on the outside of the "V". They're housed within the combustion chamber, above the pistons.