Pull the manifold, inspect the sealing surfaces, they usually leak at the center 2 ports, slightly warped manifold. Have it machined straight and true, new gaskets and good to go again.
On a second thought if you have the flat flange 340 style manifold it is easy for them to blow that gasket out at the manifold to the exhaust pipe.
Other style bell flange manifolds are easier to keep sealed, just got to keep them tight.
Car was originally a 273 car now a 318. I assume they are the original manifolds. I did a leak test and it's mostly coming from driver side nearest the fire wall. I started spraying the nuts and bolts with pb blaster for now.