If you're using the Holley 390 with vacuum secondaries, delete the power valve entirely, richen up the jets by four steps to tart with and switch to the big accelerator pump assembly. You can adjust the pump squirt with different cams to handle acceleration and adjust the main mixture by making the jets larger or smaller as needed.
The mechanical secondary is a little more tricky because you have two accelerator pumps and two more squirters. It is more sensitive because everything works off of the throttle linkage, so you have to compensate more on pump and secondary timing.
The NASCAR 390 is even a little more persnickety, but it is another animal altogether.
If your engine ever backfires with power valves, they will most likely rupture and leak or fail to open properly because of the leaky diaphragm.
