Alrighty then..I have been asked this a hundred times from many of our members...and I'll answer another 100 times if it will save someones life...
You should not mount a 200(or a 190) rear tire on the stock 3-1/2 rear wheel. Its dangerous and anyone who says that it can be done either has not done it, has 'seen' someone else do it, or has not crashed yet. That and it will tear up the tire and rear fender and the wiring that goes with it with stock shocks.
Play it safe to ride another day and do it right.
The difference between a 170 and 180 is not much and won't look that much different.
Its all about balance and our bikes are the best balanced cruisers out there.
Ride safe,
Doug