Several local builders use Ateco Automotive in Waukegan. I've had them do general machine work, etc. and they have always done a good job. You might also try Fast Times Motorworks in Skokie. I'm not sure if they do flatheads, but they're one of the best shops in Chicago area.
A real good place if not the best is Opel Engineering . It's been around a long long time and the guys were Roundy Roundy guys in the 1950's building the flatty. <ADDRESS class=adr>1538 Burgundy Pkwy Streamwood, IL 60107 </ADDRESS>(630) 837-2900 opelengineering.com