Too much gear?

I recorded the tach and A/F gauge with my phone on a run. At 6000 in 3rd gear it showed 12.5. My son had a similar thing happen with a 340 aspen, broke the 355 rear and all we had was a 486. Put a 28.5 or so tire on it. Ran the same mph as I did in the 1/4. That one had a smaller cam also advanced 4 degrees. Sold it before we could take it out with a higher gear.