The safest way is to fabricate a ram lock by splitting a piece of heavy wall pipe lengthwise and welding hinges to one side. Clamp the pipe over the hydraulic ram for the bed lift cylinder so it cannot retract. I did a lot of maintenance on dump trucks when I worked for a concrete company and we had a custom-fitted ram lock for every truck in the company. Our ram locks were made out of schedule 80 steam pipe and had hinges and welded-on flanges for several bolts to hold them clamped over the ram.
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!