Truck 261's had solid lifter cams from the factory. What makes you think it's got as hydraulic cam in it? Prying on the rocker arm for a few minutes with the valves closed will cause a hydraulic lifter to collapse, which is the only way short of disassembly to know for sure.

"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!