Re the mechanical/vac secondary thing. I picked up over hp and # torque when i replaced my vacuum secondary spring with the lightest "white" spring on my 303 motor. I have now a HP Pro 750 vac secondary Holley, and perfect drivabilty, response, and tons of power. Did not take much more than slightly leaner jets, and another White spring, and tweak the squirter spring bolt.
I have heard from several experienced engine guys that for unknown reasons, some engines seem to prefer one or the other Carb, Demon or Holley, by the way.
So i was talking with another engine guy about my "white spring" change out, and picking up power, and he said if by opening up earlier, with the lightest spring, what would happen if i could put in yet "lighter" springs, would that give even more power, by bringing in the secondary's sooner? Then would not at some point then just going to a mechanical secondary be the same as some ubelieveably "light spring"??
Sounded like a good anology. He happened to mention that his days with a mechanical showed to him that as along as he used some judgment about tromping on all four barrels at too low an rpm, he never had "bog" or driveability problems.
But for me, who is not so experienced, i can indeed tromp on the gas, and let the carb decide when it wants and needs more air. He did comment the extra gas used was substantial, and if not consumed, could possibly cause rapid cylinder wall wear due to lube wash-off, and further gas contaminate the
oil.
So i think unless you intend to run all day at high rpm, heavy throttle, a Vacuum secondary with a "white" spring is a better choice.
Like thinking about a single plane vs. a dual plane manifold. The former may give a bit more "top end" but probably gives up some lower in the rpm range, where in all liklihood a motor spends the vast amount of time.
I personally think one should very carefully think about what rpm and how the car's power will generally be used, and then select components on that basis. Just more power alone may not make for a very useful car.