I have the 2.50x16 cast wheel on the back of my RetroStandard bike, with a 130/90-16 tire.  The fitment chart shows the tire is acceptable on that narrow rim (taller profile tires allow for more fitment than short profile tires).

I have no idea what size tires are available in whitewall....that is a research project for someone else!

Buying/changing tires and wheels is a time consuming and costly project.

I can't help but think this whole thing would be far easier/cheaper to buy a set of portowalls and see how they hold up!