With the same simple independend suspension for the rear-axle as above. It is all you need. If I am not mistaken it will fit all Peter Lynn Competitions and Big Foots. Nice coil-over shock absorbers. The rear-axle is about 140 cm wide which means for high speed.
Some negative angle is then an advantage against being pulled sidewards.
Because the shocks are more horizontal then vertical positioned. The forces are not guided through the shocks but directly to the axle. The only reason for this position, I can think of, is to match the limited free travel of the shock to the required free travel for the rear-axle. This is a swing-arm contrustion. The construction is also used for motorcycles with sidecar.