Figuring out exactly where an exhaust noise is coming from is sometimes tricky. If the location of the leak is not apparent by sound alone, a first step is to visually check the manifolds and pipes to make sure there are no small cracks or breaks visible. Then, if nothing can be seen, start the engine and have an assistant run it at various speeds while you run your hand near (don’t touch!) the exhaust manifolds and, from under the car, the crossover and exhaust pipes. You can sometimes feel the hot air escaping. If still no luck, another way to possibly detect the leak is to run the engine, then pour a bit of Marvel Mystery Oil or ordinary engine oil in the carburetor and look for smoke coming out of one of the above mentioned points.
Assuming the source of the leak is still elusive, the next step is to unfasten the six manifold bolts on each side of the motor so that, using a large screwdriver or pry bar, you or an assistant can pry each manifold a small distance away from the corresponding cylinder head. Then, using a mechanic’s mirror and light, the other guy can visually check that there is no corrosion on the surfaces where the manifold mates to the cylinder head which may cause a leak that even a gasket can’t contain.
A more involved procedure is, after removing them from the motor, to check the surfaces of the crossover pipe and its flange, the heat riser and the outlets of the manifolds (particularly around the studs) for the same defects, as these are susceptible to cracking and corrosion when they get old. Also, the heat riser can develop an exhaust leak not necessarily via a crack, but through the shafts supporting the butterfly valve which extend into holes in the heat riser body. Because of this very problem, I replaced mine with a spacer. (As I live in a mild climate, my engine warms up well without the hear riser, even in the winter.)
By the way, you must have a heat riser or a similarly dimensioned spacer in its place, since the crossover pipe is engineered with the correct angles and to be just long enough to reach between the outlet of the left manifold and the inlet of the right manifold. This pipe is quite rigid, so it is almost impossible to bend it to reach the manifold in the absence of the heat riser or a spacer. In such a case, the angle of the end of the crossover pipe and its flange would not be parallel to the surface on the manifold it mates to, causing an exhaust leak.
If you still can’t isolate the exhaust leak, another possibility is that there is an exhaust leak where the INTAKE manifold’s center arm joins the cylinder head, caused by either a fault in the intake manifold or a bad gasket. This passage provides heat to the carburetor from exhaust gases to hasten warm-up (and which is regulated by the heat riser). This happened to my motor some years ago and took a long time to diagnose as it is not something one would expect.