Intake leak

Depending on how many miles your engine has on it, it may be the combo intake/exhaust gasket.
Slant Six has the unique attaching studs and washers that allow the combo intake/exhaust to move during engine heat cycles. The gasket actually wears after 100's to 1000's of cycles.
My 63 in the avatar was almost gone when I got the car and it ran poorly.
Keep a eye on where the washers go if you have to change out the gasket.
Do not over tighten as the torque is like 10-15 ft lbs or 200 inch pounds.