If $700 is out of your budget, I think you are going to have to be content with what you have. If you have a 235, it will run 60 all day long with the 3.90 you have. GMC and probably Chev also offered a faster ratio for some automatics. But they are rare and expensive if you find them. Chev pickups ran 60 and 70 all day long 50 years ago and they will still do it. You just have to get used to the noise.
