235/261 engine head isn't a good design from the point of airflow.So the power is limited. A 390 Holley should be fine for your setup.A vacuum or airflow controlled secondaries can open smoothly without the sometimes bog and then rush of mechanical secondary 4 bbl carbs.So your carb may feel "slow" but is really getting the job done.The vacuum thing is easy, the guage needle will drop to zero when you floor the pedal and may slowly rise as the rpm's increase.Like the Holly thing, you can't do this test in the driveway.
