swapping a subframe

I'll start by assuming you're referring to the torsion bar/trans crossmember
part of the uni-body,based on the " on the ground" part.Well,you need a good welder,
man and machine to do a nice job.I'ts dirty,time consuming,and requires patience.Other
than that it's easy! Seriously,I'm also assuming you meant '66 and down by "early", and
would have to be as they're about 2 1/2" narrower rail to rail up frt.
There are lots of things/methods/tricks to use when fabbing & repairing
unibodies to fit and align parts,but 'til we're sure what you're fixing.........