I wouldn't go any higher numerically than 3.73:1 for your case. I have a '77 K20 with 4:10:1 gears, no overdrive and 31" tall tires. I can get on the interstate and go 70 MPH, but she is working hard and I can push it to about 80 for a short stint, but 70 is pushing it for cruising for a length of time. I wish it had lower gears sometimes because that high RPM running coupled with the noise is fatiguing on the driver. At 70 MPH it is turning a little over 3,100 RPM. At 80 she is turning about 3,600 RPM.
With the same 3.73:1 gears it would drop it about 350 RPMs to: 2,850 at 70 and 3,250 at 80.
With 2.56:1 gears I would be turning about under 2,000 at 70 MPH, so bigger than a 1,000 RPM drop! Run your truck on the highway at 70 in 2nd gear and you will get the feel for what it is like! With 30" tires, 2.56:1 gears you will be turning about almost 3,100 RPM at 70 MPH in 2nd gear.