Have you looked for a tag on the rear end differential? I'm not a mechanic at all, but I've just started learning about differentials for my 52 F1 -It has a Dana 44 with a 3.92 gear ratio. There should be a tag bolted to it with two numbers and a line between them (Mine is 47/12)
Divide the first by the 2nd and you'll get the gear ratio. At least assuming no one has replaced the gears inside. I confirmed mine by going for a ride and comparing the tach to my speed and using an online calc. and it's a 3.92
Here's the link
https://spicerparts.com/calculators/...rpm-calculator
BTW - i have a 239 flathead. Cruises comfortable at about 60 to 62 - seems to be the sweet spot for it.