I'm thinking this is a 305 chevy engine your referring to with a .040 overbore.
First of all, why would you use big valve heads on a 305? Did you bolt the heads on the bare block to check the valve to bore clearance? the 305 bore is barely big enough to clear the 1.94 1.50 valve combination.
Valves that size on a 305 presents two problems as mentioned, one is excessive shrouding, two is the possibility of the valves kissing the bore so even if it has enough bore clearance the shrouding will kill the flow so power gains wouldn't even be noticeable. Lastly is if this engine was built for street use it uses small valves for high velocity to help it develop low end torque. The big valve kills the velocity so it would have to be revved to stink before it started to make any power and that's when it would run into the next brickwall which is the excessive shrouding.