Rear gears

[pontiacvince]

After driving my '71 Suburban with the original warmed over 307, 700r4, and 3.07 ring and pinion gear for a while, I've decided I need to change rear gears. At 70mph, it's only turning 2000rpm's. Looking at the vacuum gauge, it's struggling at that low rpm. If I had a bigger engine with more torque, it wouldn't be a problem, but the 307 wants and needs more rpm's. They sell a 3.42 and a 3.73 for the 12 bolt truck rear with the 3-series carrier. I want to maximize fuel economy, but would the 3.42 still be too low? Opinions wanted.