My son's 36 has parallel leaf springs, late 40's early 50's Olds and probably your 39 uses the 2 parallel arms and coil springs. that is the same design sometimes called truck arm. Gm used it on pickups in the 60's and NASCAR uses on the rears in current cars . If you need to replace the bushings you might have trouble finding parts . It's an OK design, on some of the olds we used to put washers under the bolt heads to be able to get more compression on the rubber bushings on the arms to firm up the ride. usually they have a rear sway bar and panard bar. one disadvantage is they used the lever arm shocks. and rebuilds are expensive. you could switch to regular tubes, look at gm pickups in the 60's for ideas for mounts , I think they used tube shocks.