"My" personal opinion on waking up an inline 6 that doesn't include a rebuild is:
Headers of your choice (I can't reccomend any, just get some
)
Offenhauser 4bbl intake (better low end torque for the street vs the Clifford)
Holley 390 4bbl (pretty much the standard when going for carburation)
RV or Marine cam (Once again, don't get me to lying on the numbers, just get one
)
200-4R Tranny for the length of the tranny and gear spacing as noted before. Nay sayers will say it is a weaker transmission=TRUE but at the same time if you build it, you can built it better than stock and plenty tough.
Make sure you've got HEI of course, stock will do, and Maybe an electric fan too...
That's all bolt on stuff except the cam.