There are lots of things to think about when it comes to an upgrade of this sort, and obviously, cost is an issue. As you mentioned, one option is finding a suitable replacement. Another is having your original rear redone. The price comparison can be made by calling a few local rearend shops and wrecking yards in your area for quotes on both. But in order to get accurate pricing on used rearend assemblies, you'll need to know what you're going to swap in. This is where things get a bit dicey. In order to decide what's going to fit (widthwise), you've got to figure out what you're going to run for tire size and width, rim widths and backspacing, and if you're going to slam the truck or leave it stock height. These factors will determine the rearend width you'll need. You'll also want to think about whether or not you want to retain six-lugs or swap over to a five-lug setup. Any '65-72 Chevy truck housing is a bolt-in ('65-69 are all the same width), but widths changed with the '70 model year, and after '65, the lug pattern changed to a five-bolt pattern. Once you've taken a couple of measurements and have decided on the width you'll need (and if you want to change the lug pattern or not) you can then get some price comparisons between having your original "one-legger" converted to a Posi unit or swapping one in from a later-model Chevy truck.
As far as the ignition system for your small-block is concerned I personally think that for a "street" engine the factory HEI would be more than adequate. If you think you'd like a bit more muscle in the system (perhaps for an occasional blast down the strip) then by all means check out what the aftermarket has to offer.