The SM420 is the original transmission. It will fit your truck like it was made to be there. The SM465 is the later transmission, started in '69. I recommended it because it may be easier to locate than the 420. Both transmissions were pretty rugged for daily use and they're a virtually trouble free swap in our old trucks so you could start with the 465 and if you stumbled across a 420 you'd have a good replacement.
There is a heavy duty 5 speed called an NVG4500 but it's not an easy install with the factory bellhousing. There are a lot of differences, some subtle and some not so subtle, between the front of that trans and the 420 / 465. The better route by far when installing this trans is to use a conversion bellhousing Originally the Chevrolet version wasn't equipped with a speedometer cable drive but the aftermarket has come up with solutions for that.
Note: Research this AM has revealed that guys are using a Dodge version of the NVG4500 and a plate that sandwiches between the factory GM bellhousing and the Dodge transmission.
http://www.advanceadapters.com/products/pn-712550/ . You could keep your factory clutch linkage that way.
But the 4 speed and new rear axle is still probably the least expensive and shortest route to getting on the road. I think a 9" is a good choice if you re-drill the 5 lug axle to 6 lug. Around here I can find pickup truck axles for $200 to $400 in junkyards and less if I search Craigslist. The transmissions are almost throwaways since people seem to think 5 speed is the only way to go.