I kinda want to do a turbo 300ci I6 one day. They are tough engines, 7 main bearings and stuff. 7,000RPM isn't out of the question, the trick is getting it to breath up there. Check out Clifford Performance for overpriced I6 parts.
There was a dude selling aluminum 250ci (if I remember right) heads on ebay, they're from Australia, but are actually OEM Ford parts. Yeah, the aussies get a lot of cool stuff we never even hear about. There's a quote from some Ford US exec saying how they just blew a huge pile of cash developing the V6, and the old Ford Aus. inline 6 would beat the snot out of it.