No board trackers are comfortable to ride. If you look at motorcycle photos from the same period, they NEVER used upside-down handlebars.
And some of the "normal" handlebars were not that great either.
I built a vintage-styled motorized bike that had long Shelby-style handlebars, I think they were about 26 inches long total. The dimensions were accurate; I took them from a page of an old catalog I found online.
The bike was comfortable to ride once you got underway, but the low-speed handling (like, at walking pace) was absolutely terrible. Making a tiny steering correction involved shifting the grips six inches left or right. I had to gun the engine when starting to get moving quickly, as fast as possible. I simply couldn't balance on the thing when riding at low speeds at all.
http://www.norcom2000.com/users/dcimper ... cycle.html
-------
I used a cheap bobber motorcycle seat for mine. It was very flat (with the upward bend at the back) and wasn't really very comfortable. Mine cost $65 but the better bobber seats are up around $200.
The problem I found was that the motorcycle seat could have been comfortable, if you could have located the footpegs correctly, but I couldn't do that because of the way that the whole frame ended up being built. The cranks were in the way, and so one of my legs was always forward, and one always back, and one leg was always uncomfortable due to the flat edge of the seat.
If you're building a frame from scratch, one thing I would presume from all this is that a motorcycle seat wasn't very comfortable for pedaling and I already knew that a bicycle seat isn't going to be very comfortable for just sitting on (while using the engine).... so you kinda need to decide what way you're going to move most of the time, and build the whole bike around that.
I don't have that bike anymore, I threw (most of) it out. I kept the engine and wheels.
I had a lot of fun building it but it had a lot of mistakes I made, I didn't have any metalworking equipment when I built it and it couldn't climb hills at all due to the engine's single-gear limitations. And finally, I lost the storage space I was keeping it in.
....I will probably build another one eventually, but don't have any idea when. I want to make the engine myself, since I don't like any of the ones available. They are all high-RPM, which means vibration and buzzing which was another aspect I hated. I want to do a cross-head type engine that will spin at much less RPM, and have much less noise and vibration. No company anywhere builds small engines of this type; if you want one you pretty much have to make it yourself or pay somebody to do it for you.