making my truck drivable

[jewels.]

Hi guys! I haven't posted in well over a year. But my daily-driver
car broke down, and I am now forced to get my chevy drivable.

I have a 1968 C-10 Stepside.
It originally had a straight-six under the hood.
(I replaced the six when it blew with a 350 V-8 small block.)
The truck has a three-on-the-tree shifter.
(Three speed on the column.)

The problem is that the truck tops out on the highway at about
55 or 60 mph. It is just racing at that speed.

After searching the boards, I found two possible improvements
that I can make to get more speed out of my truck:
1. Increase the tire/wheel size. (I am in the process of digging
up some original rally wheels to replace the small rims on her.)
2. Re-gear the rear end by changing out the ring & pinion (or the
full rear end).

My question concerns #2. I was trying to find out my current
gear ratio. After searching the boards, I found three possible
ways to determine my gear ratio:
1. Count the teeth on the ring and pinion gears and divide.
2. Read the stamped gear ratio off the side of the ring gear.
3. Rotate a rear tire and count the simultaneous rotations of the
drive shaft.

So, I didn't want to have to pull the diff cover quite yet
(I just did that recently anyways)
So. I tried #3.

When I turned my wheel:

1 full turn of wheel == 1-7/8 turn of drive shaft

Does this sound really super low?
What is wrong here?
I thought that if I had a 3.73 gear ratio, I would get 3.73 turns
out of the drive shaft for every turn of the wheel.
Is my truck geared really low?

Any suggestions you have on this, or anything else I have
commented on above would be greatly appreciated!

Thanks so much!

jewels