Cleveland heads have always been renowned for their bad exhaust port design. I remember reading years back something about it being a design compromise to get the exhaust manifolds to fit in between the shock towers. Ideally they wanted the port to be much higher.
Racers used to machine the whole exhaust port off the head ad replace it with a new aluminum plate with raised exhaust runners.
Gains can be made by putting port tongues in the exhaust ports to lift the port floor and make it D shaped.
There's huge potential in the Cleveland heads but you guys in the US didn't get to see much of it as the motor was dropped in the early 70's. Australia kept going with the 351C in Ford sedans right up until the mid 80's. As a consequence we now have companies like CHI and others making some amazing heads for these motors.