I have a 67 Plymouth Belvedere and I have a couple of questions. First, I was reading this article that said there are a couple packages that I can add to the engine to boost MPG and HP. The packages are: HyperPak and the Feather Duster. Now I dont know if these are aftermarket packages or packages that are only available when you buy the car. Heres the website where I got these packages from:
http://www.allpar.com/slant6.html

 

I believe that those were only offered when you bought a car.

Clifford Performance has quiet a lot of slant 6 parts though that will allow you to really make a fire breathing motor.

 

those were packages offered by chrysler back in the 60's and 70's
there are things you can do to the /6 to get better mpg

what are its stats?