Narrowing b-body axles

No go...

Take 3/4-7/8" off the end and that's where the spline needs to start. You'll be into the necked down area (taper) which is smaller diameter than the OD of splines required.

You'll be buying new axles. 68Dodge nailed it. The only axles that seem to not have the neck down are truck and C body pieces.