Rather than cut the frame directly through above the axle I staggered each cut by several inches.
I used 6″ steel pieces each cut.
I prefer these older frames as their box construction makes them stronger than the newer design.
Prior to cutting off the from of the frame I added cross bracing to keep the frame square. They also provided a great place to place jack stands.
I used some scrap I had laying around with old spring mount bolts to extend the frame exactly the right amount.