74 225 slant 6 options

I am in the same position as the original poster. I don't really give a hoot about the matching numbers, a slant 6 is what I have in my Duster and I can't quite afford going to a nice 340 or 360 built. Anyway, where can I acquire this super 6 upgrade? I also noticed a header on JEGs for the dodge I6--nearly $700! Ouch.