It's the cam more than anything holding you back... Not only the heads IMO. That's no duration for a 4" arm and I'd expect it to shut off at 5500. You have to keep in mind that these have 8% more stroke than the 440. And the stroke is what gives the engine its "spirit". A 241 @ .050 cam in a 440 is a mild to medium street cam. I normally use cams in the 250 @ .050 range and those power curves peak and drop around 5800 and that's with 870cfm wet flowed carbs (they are not under-carbed). While I agree with 1Wild says with them being too small, you have left a lot on the table with that camshaft. If you want to make power at 6500, you do need more head, but you need WAY more camshaft.