I've had several flatheads in my time, and personally think they were the best sounding engines ever with headers and Smithys. They were state of the art in their day, and had plenty of torque and power for the cars and trucks we were building then. I have a complete 59AB in my shop just quietely waiting for a ride.
As far as your project....I think I'd put the flatmotor back in the '31 and preserve a piece of history, then find a 292 to hotrod and put it in your ride. A warmed over 292 with proper gearing can be a real sweetheart. If you get a chance, check out "Chevrolet Inline Six-Cylinder Power Manual" by Leo Santucci.
Just my $.02
