Jack up one wheel. Mark the driveshaft and the wheel. Make 2 complete revolutions with the wheel & count the turns on the driveshaft. It should be fairly close. The stock ratio as mentioned is 3:90, if it came with a factory overdrive it would be 4:11. Of course anything could have been changed over the last 50 years.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine