On your 440 the vacuum secodnaries would have opened all the way at WOT if it had been set up right.
The way the come stock is very conservative (slow secondary action) becasue most cars accelerate slow.
You can change that but it takes specific re-work. It is actually more work than getting a DP carb right.
vac secondaries do not work on manifold vacuum at all.
They work on the vacuum created by air flow thru the venturii. There is a signal port on the pri and on the sec
venturii for this.
manifold vacuum has nothing to do with it. There is more to it than just swapping the spring in the vacuum diaphram housing. You rarely need to remove the check ball. But the bleed under it can be adjusted for bleed flow. It is setup very conservative.
The bleed can be made adjustable by adding a orifice needle screw. (like a idle mixture screw) on the vac sec housing.
You can modify the stock vac sec housing or buy a special adjustable type.