IMO, the best swing axle trans starts with a 71-73 double sidecover IRS. That way you get the better guided throw-out bearing, and the bigger pinion bearing. If your trans is an earlier IRS trans, its no better than the last swing axle trans.
The only advantage to short axles is fender clearance with the wheels and tires you want to run.
Theoretically, a longer axle (all other factors being equal) will wind up more than a short one. If the axle winds up more, it will be absorbing more of the shock, and thus should be easier on the trans parts.