Got a good deal on a 10-bolt, 8.5" Posi from a 1977 Elky. Need to know if anyone here can advise me on swapping that axle assembly into a 5th Gen. I know it's a little wider and the trailing arms won't work. Seen many posts that mention special trailing arms for this swap but no one ever mentioned where they can be bought. I have adjustable upper control arms with a Heim joint, can I reuse them? How about the driveshaft?, needs shortening?

I don't know what the control arm geometry is on those years. I know from like 67-72 you need the upper control arms that adapt the different angles but know/haven't heard anything about the 73 to 77 years. If you have heim ends on the chassis side, you could probably try the billet spherical upper housing bearings that trz and wolfe race craft make:

http://www.wolferacecraft.com/images/shericallg.jpg
(wolfe race craft)
Wolfe Race Craft shperical upper bearings (http://www.wolferacecraft.com/detail.aspx?ID=187)

I couldn't find a picture of the trz bearings, but they are the same except for the hex head on the ends.

The heim ends and the spherical bearings should take up any misalignment between the suspension geometries and you would be able to reuse the uppers you already have.

As far as the lowers go, they might be the same, I know they are on the 67-72's. You might also have to check into the spring perches and shock mounts.