Oly, the 19.5" is a tubeless height equivalent of an 18" tube-type tire. I doubt if you'll be able to find a 16" tire tall enough to match, so your road speed will drop, and RPM will probably go up noticeably. Is the truck a dual-wheel rig now? My 59 model 3800 flatbed had big enough rear brake drums that 16" wheels would not fit over the drums, so I would have had to go with outside wheels only if I'd swapped to 16's, losing the dual-wheel capability. Something to consider- - - - - -
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!