I think the cam is too big for the rest of the build. The old MP solid lifter would make more than 550 hp in an 11:1 440 with ported heads. I don't have any experience with those heads but my guess is that they aren't a good match for that cam on that engine. It is a curious result, I'm not 100% sure why it isn't making more power but my guess is that the big cam is killing power down low and the heads are killing it up top.

Running a cranking compression test while it is on the dyno and see what it says. Look at the plugs to see what they say. If the engine is in good mechanical shape then I'd suspect a mismatch of parts. I'd go about 10 degrees smaller on the cam if the heads you have won't flow up top. Then it will at least be a fun street engine. If you want a race engine then you'll most likely need to have the heads looked at by a pro.