Definitely don't overdo it on boxing. When I was building dirt track cars from tri-5 Chevy frames, we did a lot of boxing- - - -and a lot of crack repair. We needed a stiff enough frame to hold jackscrew suspension adjustments, but even building the roll cage like a bridge didn't eliminate frame flex. You're not going to, nor do you need to make the frame totally rigid.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!