frame boxing

I received my ABC Perf frame kit and had a few questions for those who installed them. Particularly the side rail section where it meets the body bushing bracket. The issue I'm having is if I cut back the bushing bracket flush with the edge of the rail, the precut ABC Perf kit appears to be...

I'm in the process of building a chassis form the ground up. I'm working on the frame now and just made templates for boxing the C channels on my 64SS. I had the frame blasted and looked at the condition of the original factory welds. Amazing the thing held up all these years. Nothing cracked...