well gt40 heads are not all just gt40 heads. they are different some have a different exhaust setup so it requires a different header. the spark plug is at a different angle. some have smaller ports. some have smaller cumbustion chambers.
as you can see the gt40 doesnt describe enough.
they make after market casting gt40 heads and they are different too.
ooo and gt40p heads add about 20whp to a stock 5.0 so its worth it if they cheap