Tach Install Where and Whats The Trucks Redline?

[man7sell]

Driving the latest acquisition, I find the truck revs too high for the speed it attains. It's a manual 4 speed. First gear the revs are high and the wheel turn about half way around so easy to take off in second then quickly to third and now I'm doing 20 mph ready for forth. At around 40 MPH it seams to be pretty high revs.
So number one, I'm fitting a tach. Got it all hooked up except the green signal wire. This wire will go to the distributor, I think there is a tach connection there right? It got too dark to continue last night.
What should I expect for revs at a cruise speed?
Took a pic of the rear axle diff. Note on the right some suspicious numbers on the right axle. I think this axle has been replaced with one from a wrecking yard. Probably with the wrong ratio.
Not a manual transmission should be on to one in forth gear or am I totally off base?