Slide two or three 3" diameter solid steel round bars into the C channel of the frame rails under the bed, long enough to stretch from one side to the other. Be sure to bracket them in well so they don't come loose in case of a panic stop or a fender bender. I did something similar with my son's 72 Dodge pickup several years ago with a piece of 3" channel iron and two cast steel track idler rollers from a D-6 Cat dozer bolted to it. They weighed almost 100 lbs. apiece.
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!