Turbo is probably the best route.
+1 for what Wsovonick said puls check out the forum at fordsix.com. A ton of information on modding the I6.This manual has lots of ideas on beefing up your Ford I-6, based on your ability and budget. Don't be misled by the title (Ford Falcon Six Cylinder Performance Manual), it applies to all the classic Ford sixes; the 170, 200 and 250.
Classic Inlines Performance Parts - Ford Inline Six Products