Here is an article by Jan Heine regarding the upright position: https://www.renehersecycles.com/myth...e-comfortable/ It explains what hokiefyd said in more detail. There is an advantage in building from the frame (e.g., Buena Vista which he suggested) -- you can leave the steerer longer than you need keeping the possibility of moving the bars up and down depending on the type of riding you do.

You can mount thinner (or thick tires with less rolling resistance) tires on a bike like Specialized Roll, but this is only a part of the equation...