If you can, it's better to reinforce the rear frame area where the bar attaches - have a friend whose u-bolts ripped out the frame under cornering forces. He then had plates welded to the underside of the frame and used the original mounting method. Remember our frames are not very thick to begin with.
Alternatively, instead of using their u-bolts, use square u-bolts with longer legs (or two hex bolts) and drill all the way through the frame from the top; then use a 14ga. - 1/8" plates top and bottom, through which you insert (recommend tack weld the top) the bolts to spread out the forces.
My 4A has a front 7/8" Addco bar mounted to a TR6 radiator shield. It uses different holes than the factory 6 bar but fits well.
hth