A little preamble in the hope that there won't be misunderstanding or hurt feelings. The first sentence of the next paragraph I have used in snark and in earnest (as all good uncles should).

I can teach you but I can't learn for you. Human nature seems to crave silver bullets and/or magic pixie dust. There's not going to be a single, easy to communicate answer to your question. I happen to be in sync with Roger's answer, but there are years of understanding behind that concurrence and a desire to expand on it. But then I think about the complexity of the answer and potential outcomes when it comes to the whys and wherefores based on accumulated knowledge/experience. Those who've got that sometimes take it for granted.

At this juncture in your learning curve I would advise you not to be in a hurry to buy an axle just because it's swap meet time. Whatever motivation you impose on that notion be it price, availability, whatever, axles are such a common commodity for hot rods that there's rarely an opportunity for a "score" however you define that. There is such a range of paths to follow to get to your stated goal that you've got many more decisions to make before zeroing in on the specific axle choice regardless of it's configuration.

Since you've realistically set next winter as your build start, and given that whatever your choice of eventual axle is will be readily available, I would suggest your first goal is to spend the upcoming car enjoyment season learning what this older hot rod (conceptually) stuff is all about before throwing money at parts (I think that's what you've acknowledge above). Observe cars at events, seek out builders in your local area, ask questions here or wherever you hang once you've seen something that needs embellishment in your mind (questions like "why did so and so do that..." are most times easier to answer than the wide ranging one above). The point of the first sentence in the second paragraph is, you'll learn more of value once you observe enough (or attempt enough) to frame a context.

The following is observation not criticism. You've posted on here before about bodies and quite frankly you've been all over the ranch, especially with the body ideas expressed in this thread vs. previous. That's not a bad thing, it just shows you're gathering info and your brain is processing. But for those trying to help, it's also confusing. Observe, process, learn. But be mindful that those giving you advice like to be reassured that you'll take what they say to heart in some positive fashion or other.