'71 Rear Axle question.

[71C10SWB]

Hey guys,
I hope I'm not beating a dead horse here, but I've got a '71 swb that originally had a 6cyl/3-speed. Now it has a 350/Powerglide (soon to be a TH350). The rear gears are really low.
My father-in-law has a '69 or '70 (350/auto) down in the woods that he said I could get the rear out of if I want to. It has the 6 lug wheels though. I thought I would just swap axle shafts when I got it to my 5-lugs, but from what I'm reading here, I might not be able to do that.
He also has an old mid-70's truck down there. I know it has the leafs vs. my coils, but I don't think it would be a real big deal to convert.
What would be my best choice here on a budget?

By the way, does anybody know the axle width dimensions for both versions?
If so, is there any external signs to tell a narrow 67-early 70 from a late 70 6-lug?
This thing's down in the woods and it's pretty wet right now. It would be nice to know before I crawl around underneath it.
thanks