How much gain with lower gears?

Here are 2 pure on track weight/mph examples>

My stock '71 340 Cuda, 3.55's x 26" slick, 3500 verter, .484 purple cam, went 13.3's@101 in street trim, trapped@5700 = 301hp...added>
.590/312 Solid ft cam, more carb, HSD intake. bigger hdrs, 4.30's x 28" slick, 4200 verter, went [email protected], trapped@6300 = 367hp, so +60hp is approx 4/10ths, the rest was gear/verter, 60ft.

Opposite example>
'69 440 Cuda, best n/a with 32" slick, 4.88's =10.7@125, trapped@6500,
+N02 went [email protected], trapped@7350, not good with 6pk rods/pistons, back off the gearing to 4.56/4.30 or tighter verter with less slip, or both.