The flanges won't matter very much and there's no way short of cutting and welding to get them perfect. I wouldn't fight it. Get the roll bar to fit side to side so they are close to the edge of the longeron. On the slider, the side skin fits right up against the side so you want them close but not past the edge of the longeron. Once you get there, drill and install temporary bolts but don't cut the forward roll bar brace or bolt anything permenant. Reason is the slider frame will determine the final position of the roll bar. You may need to shim a degree or two forward or aft to match the canopy frame.
Estes Park, CO
wirejock at yahoo dot com
Donated 12/01/2021, plus a little extra.
RV-7A #73391, N511RV reserved (3,000+ hours)
Empennage, wings, fuse, finishing kit done. Working FWF
I cannot be, nor will I be, held responsible if you try to do the same things I do and it does not work and/or causes you loss, injury, or even death in the process.