Nothing wrong with the rivots. If your spring hangers are damaged or loose, go ahead and replace the hangers and bolt them with grade 8 bolts.
There are plenty of online gear calculators out there where you can plug in the known variables to see rpms at a particular speed.
Low gears pull a load better, and accelerate better. High gears just don't push you in the back of your seat. Generally having a final drive ratio of around 3:1 is on the economy side. Having a final drive of around 4.11:1 is on the performance side.
Using lower gears (4.11) with an OD transmission will put your final drive ratio close to 3:1. You will not lose the acceleration the lower gears offer, and you will get the economy of the higher gears. The OD transmission really is the best of all worlds.
Tire size can also affect the gearing. 31 inch tall tires with 4.11 gears are equal to 3.73 gears with 28 inch tall tires. The 4.11 gears are lower than 3.73 gears, but with taller tires, the will behave like 3.73 gears.
I am a big fan of lower gearing. In my 59, I changed the factory rear to the after market 3.38 gears. 3.55 gears are not available for the 1955-1962 1/2 ton rears. When I blew up the factory rear, I wasn't too upset because I knew I was getting posi. I went with 3.7:1 gears. Those gears worked great with 28 inch tires and OD. I went to taller tires to give the truck the stance I wanted. Now I want the 4.11 gears to give me the acceleration I lost with the taller tires.
There isn't one answer here that will work for everything. Play with the numbers with a calculator till you are satisfied. There are several trade offs that need to be considered.
