Here is a factory 19 front and chrome 16x3 rear combination for reference, on an early K1 with an F tank on it. It was a popular swap at the time as you could put a much fatter tire on the rear yet retain the factory front and more or less blend in. Very common with vintage sohc4 choppers as well.
There can be an endless debate regarding how such rim/tire size changes will affect your bike's handling. For you it matters how you intend to use the finished bike. Clearly, if your interest is to try to hold your own in a steep corner with a modern bike, that combination probably isn't ideal.
The finished tire size on each rim also affects these things. Tires for 16x3 rims tend to be wide and reasonably tall, not too far off a factory rear tire. You can get different diameter tires for an 18" front tire, varying by more than an inch.
Most that use a 16x3 rear also use a 19 front, but nothing wrong with a 16/18 setup.
George
