Are the rear arms from all years the same/ interchangeable? My car is a '70, and my spares that I've been rebuilding, strengthening, welding tie downs on, are from an unk yr. Hope I haven't waisted my time.

They are all interchangeable.

They are interchangeable but not the same. Early arms have a shorter bracket for the brake line that uses different mounting hardware and flex lines. Use the matching flex lines. Early arm brackets have a hole and late arms have a slotted hole. They also lack drain holes that later arms gave.

Early arms also have a solo tab welded on the left arm. This is because the arms were shared on the 914-6 and that tab (along with a blue cloth strip and a screw) held the handbrake cable away from the heat exchangers.

They all work though, just remember when you need new soft lines to get the early models or the late... depending.