They hadn’t figured out how to manufacture slicks that worked yet. For slicks to work correctly, the carcus needs to be fairly thin to keep the tire from overheating.

It's considered a 180 degree V-12 because the crankshaft design uses shared connecting rod journals. To be considered a flat 12, each connecting would have it's own journal on the crankshaft, which would add quite a bit to the length of the engine.