why IRS over swing

[chigger]

Welcome to the madness.
An Irs trans can be adapted to a swing axle pan or an irs pan can be installed on a swing axle car.
Here are some pluses
Stronger
more gear ratios
new parts availiable
different torsion bars available
various sway bars
handle better IRS keeps the wheels flat on the pavement better, it doesn't change the camber near as much as the wheels cycle up and down.
While the swing axle can be made to handle fairly well as it has some of the same options available as those above for the IRS for me the biggest item is the availability of parts. These babies are getting old and many parts may start to get hard to get.