Gearing as always depends on the following things:
Engine RPM range and torque curve
Expected speeds
transmission ratios
Rear wheel height
Old cars and trucks had 3.5-4.3 ratios, and were not expected to go 65 MPH all day long.
More reasonable ratios are 3.0-3.5, depending on other factors listed. There are online calculators to figure this stuff out.
Any more modern rear will have to have the spring perches cut off and the old axle's or new perches welded on. Yours should be ~2" wide and modern stuff is 2.5". The U bolts won't align properly unless the plate, spring and the perch are all the same size. Many have used late 60s B body axles (early have the bad axles you have now), late 70s full size, and I 've heard later model (no year range) Jeep Cherokees with discs are also good. Look for a width that is close to yours. If you have the wheels and tires you plan on running, mount them up and see if they need a little room in or out, or are okay. You will have to set the pinion angle when welding on the new perches, which is a whole 'nother arguement. Do a search on that.