i will use my friends car as a sample for you as the car is almost the same as yours.
he has a 1973 Camaro Z28 RS with stock 350 motor and a 4 speed,about 300 hp stock or less.only mod is headers.
he ran 3:31 before and the car ran like a dog.
we put a 4:10 gear in a few years ago,car runs like a champ!
with that little small block you need the low gear ratio of 4:10 to get that zip out of that low HP motor and my buddys car isnt all that bad on the highway as the small block handles the higher revs just fine.
he does have the factory taller 15"x7" rims though were you have a 14"x7" so you will rev a touch higher than his 3000 rpm. i would guess about 3200 rpm when your done at 60 miles per hour.