[wilf leek]

The way to see what is happening is to measure and look at intake manifold vacuum. If, at high revs and wide open throttle, you still have more than, say, 1-2" of vacuum, then your carb and/or filter is strangling the airflow.

So buy/borrow a reasonable quality vacuum guage, and tee it into a port on a manifold runner (not on the carb) and see what's what.
Once you have sorted that, then you should go get your carb set up properly. 750cfm carb on a "stock" 428 should be OK.