Don't even consider running a swingaxle if you have a choice. They can be evil.
I would consider '65-'66 to be the best years. '67's have a lot of unique to them parts. Earlier will work, 68's and later are IRS stock, but are the Ball joint front end. The best thing I've seen for those are the Thing spindles. They are effectively a lift spindle. Unfortunately they use a unique to them ball joint (can't recall if it's the upper or lower), if you opt to go this route get the whole Thing beam as it will bolt into a BJ car. And modify it as Wayne suggests.
You can buy the inner trailing arm brackets to convert a swing axle to IRS. You will need the IRS spring plates (& torsion bars if not the same length - there's 3 different lengths of T-bars), the complete trailing arms, all of the CV joints & halfshafts, the inner trailing arm pivot bolts, the IRS transaxle's side plates and diff or the whole IRS transaxle. The most ideal donor vehicle for these parts is a Thing. They have the Bus 4" CV's already. The std passenger cars all use the wimpier 3" CV.
Only problem with the Thing parts is that the halfshafts are a slightly different length than what you're likely to need. The most desireable parts off of a Thing are the stub axles and the transaxle diff's output flanges. Those are what it takes to get to the 4" CV's with stock parts. The halfshaft splines are the same for both the 3" & the 4" joints. The Thing rear drums are wider and result in a ~3" wider rear track width, but they don't to my knowlege have any better brakes.
Bug IRS trannies typically come stock with a 4.12 R&P ratio, while Swingaxle trannies come with a 4.37 R&P ratio. In the after market you can find R&P ratios from 3.88 to 5.14 I've broken my share of the 5.14's, it's easy to do. If you think you want that low of a ratio go with an early IRS Bus trans ('68-?) as they have a 5.38 ratio or a really rare 5.42 ratio.
There is a change in the number of ring gear bolts that predates the change to IRS. So if you have a late Swing axle car you can bolt the existing ring gear to the IRS diff w/o trouble.
To stay 5 lug you need to either have some rare parts or do some parts swapping. If you scored a Thing set of complete trailing arms you're there already. Otherwise you need to remove the spacers from the swingaxle's wheel bearing region. Can't recall which one of the two it is, but one of them will slide onto the IRS stub axle after the swingaxle's drum and space the drum nut properly.
TS
I used swerve around my halucinations, now I drive right thru them.