The square tubing idea is a good one- - - -welding plates on the ends of the tubing that can bolt into some of the existing rivet holes seems to be a good compromise between accessibility and frame reinforcement.
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!