I'm sure you've heard the horror stories of what happens to a broken axle on a jag rear end. The axles are working in torsion and compression. If you're going to modify them make them failsafe by having the shaft tight inside the tube, see
http://www.erareplicas.com/427/shorten.htm
That way if it breaks the welds it will spin and not come apart.
The ends of the axle should be the same length and weight so you can't put them in backwards. Make sure it is turned to diameter first! You can't do that afterwards unless you build some fancy yoke fixture. The tube area should equal the axle area. I'll measure some bits when I get home. Now for the dogbones...........( lower arms)...