They put a Paxton blower on Hornet inline six flatheads in order to keep up with flathead V8s. I'm pretty sure stock inline six flatheads don't breathe very well. From what I've read, those Hornets did well on circle tracks back in the day.
Forced induction will make up for lack of breathing to some extent on any engine. You then just have to worry about if the rest of the engine parts don't come out the sides or through the oil pan.
