rear end swap
 

OK, Experts! Now that I've been driving the truck this week with the tach working, it has settled down and I believe it is accurate now. Idle at 800, 55 MPH is 2800 RPM.
The truck is a 71 1 Ton Longhorn, rear end has 14 bolts. Using the formula to determine gear ratios that I've used for years, I believe I now have 4:56 ratio.
What can I switch to that will get me to about 3:50 to 3:73.
I have 8 lugs now with 16.5" wheels and tires.
I hope that either I can buy gears to put in or find a direct bolt-in to swap and keep 8 lugs.
(Randy, I bet you know!)

You might get lucky and find a Dana 60 with 40" spring centers and 3.54 gears, but most likely the 40" are all going to be higher numerically 4.10 to 4.57. (I have a 3.73 geared 40" 60 I'd swap for a 4.10 geared 40" 60 to anybody in the central TX area) 4 wheel drives, Longhorns and Blazers all had the 40" center axle housings. Most big block trucks with a 60 came with 3.54 gears. A 3.54 60 with 42.5" centers (127" wb 3/4 ton truck) shouldn't be too hard to find. You could swap the parts into your housing, but Danas use a case spreader to install the differential, and it is difficult without it. Also, if back lash needs to be changed, the 60 has the shims BETWEEN the bearing and the case, not between the case and the housing like a 12 bolt, so a bearing separator and puller are required to make adjustments.