So far as axles & need for SuperDiff are concerned, if you aren't ever going to do any wheelie/smoke-show attempts (and you are running street tires) you aren't likely to break them. As the old saw goes, some people can break a crowbar in a sandbox - like my son
![Wink ;)](./images/smilies/icon_wink.gif)
With the 4.125 R&P the only way you can attain "Freeway Flyer" status is to use a Bus .82 4th. There are even taller aftermarket gears available, up to .7, but you'd have a HUGE spread from 3rd - it'd be like driving a 5-speed with no 4th, can't recommend it.
HD sideplate is a good idea on the ring-gear side but IMO it's pointless at the level under discussion to fit two.
Just in case you're still considering a spool (or welded-up diff), those are both fine for race use and eliminate the possibility of scattering a differential - but for everyday street use FORGET ABOUT IT! Can turn a routine low-speed turn into a white-knuckle experience.