One thing to remember about a roll bar vs roll cage is tubing diameter. With a 6 pt bar the bars (atleast the main hoop) is required to be larger in diameter than if you have a entire cage. I forget the actual tubing size, but if the cage was a kit for a 10 pt cage than cutting out the bars may not make the 6 pt portion legal.
Might be wrong, don't have rule book in hand right now. Mild steel 1-5/8 x.120 dom, CM .083 x 1-5/8, welded seam type 1-3/4 x .134 wall. Side bars, if X, then 1-1/2 x .065, if single, same as hoop, seat bar 1/14 x .065
