Help me pick a gear for my combo

I'd go with a 3.91, maybe even a 4.10. I agree with Joe for once thsat an overdrive would be ideal. Plus, your 2,500 rpm converter with that engine is going to be mismatch with lower numerical gears. Small blocks need gears to get going and that converter aint gonna help. Its probably covered up right now since your rpms climb pretty quick with the 4.56 but if you change to a lower numerical set, you will probably notice a difference.

I currently have a 4.30 gear in my car, 28" tire (275/60/15). RPM @ 55 mph in high gear is 3,000. Previous owner built it for the 1/8 mile. Switching to 4.10 gear sometime this week. I drove it on the highway once just to see what it was like, it sucked. It was lugging when I was on and off the gas and it was hotter than usual. Its not set up for cruising so I didn't care but it was slow lane all the way.

Use a gear ratio calculator to see what different ratios will do.

http://www.wallaceracing.com/calcrgr.php