The 1/2" difference in diameter isn't the problem- - - -it's the drop center on the rim for mounting the tire without a tube. The 20" wheels have a straight center that slips over the brake drum. The 19.5's probably won't have a big enough center hole below the drop center for the inside wheel to fit over the brake drum. When I swapped a GM 14-bolt rear end into my Ford F-350 I ended up having to run tube-type wheels as inside duals because the brake drum diameter was not compatible with the Ford tubeless rims! Check it to be sure before you commit to the wheel swap!
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!