I’m not sure what to make of that chart. A C10 with 4.57 has 1000# more towing capacity than a big block C20?
A gear change I think would help you a bunch. 10% more torque at any speed makes a noticeable difference. Like going from 350 ci to 396. Or from 396 to 454 ci.
**EDIT** since you have 3.54 gears, it would be 15.8 % increase in effective torque at same highway speed. Remember that torque is multiplied by the gears and hp is mathematical function of torque. So if you are turning 2500 rpm with the 3.54 gears and the 4.10 gears put you at 2900 rpm, you get to now multiply the higher 4.10 number by the higher torque output of the engine. Since we can’t see below 3000 rpm on the charts posted we can guesstimate. I found a 325 hp 396 chart (which I think is same as the truck 310) that showed torque to be 375 at 2500 rpm and 410 at 3000. So let’s pretend same at 2900… 375x3.54=1327.5 effective lb ft versus 410x4.10=1681. So 1681-1328 (I rounded) is 353 ft/lb. 353/1328=25% increase in available torque to do work. Here’s the graph I found:
https://www.automobile-catalog.com/c...e_4-speed.html
It is certainly the easiest way to get more effective power. Torque is a function of displacement and compression ratio almost completely. And it’s hard to bump the compression on a big block that has 100cc heads without changing pistons.
A cam swap can move the power around but mostly trades RPM for HP by shifting the scale up the chart.
For a low performance application, towing is one place where small, long tube headers can really pay off by increasing torque without moving the scale.