Whining Noise When Letting Off The Gas

I'm not sure which type you have. I know the manual trans stuff changed in 1966. Not sure about the auto trans stuff, but I know the 66 & up manual type is the same as the later auto trans stuff, was in use for a long time & is easily available. You can find charts on line that will tell you how to figure what gear you need if you use the later style.

Assuming your speedo read correctly before the gear change, here's an example of how to do the math.

Old gear ratio =3.23
New gear ratio = 3.91

3.91/3.23 = 1.21
Now, count the number of teeth on your old speedo drive gear. I don't know what your is so I'll use 30 as an example.

1.21 x 30 teeth = 36.3, so you need a new speedo drive gear with 36 teeth. If you change to a taller or shorter tire, you use the math to determine the % difference in diameter & then use that percentage to determine the new gear tooth count.

Just remember; A numerically higher gear ratio in the axle or a smaller tire diameter will result in the need for a higher tooth count on the speedo drive gear. This is because the drivetrain has to turn faster to go the same speed, so the speedo has to be slowed accordingly.