Well, using the calculator below, it looks like with 3:07s you’d be running approximately 76-77 mph at 2400 thru 1:1 high gear.
With 3:42’s you’d be closer to 69-70. This is all estimations though because the actual tire size is going to change the results. That’s using a 33” height but at half tread your not likely going to be as accurate. Looks like its possible it may have 3:42, but I’d bet you’ve still got 3.07’s and here’s why, torque converter slip in that th350. If you had a manual transmission I’d say 3:42’s are likely, but with the auto it’s normal to run a few hundred Rpms higher than a manual transmission with the same gearing setup.
https://spicerparts.com/calculators/...rpm-calculator