gear ratios?

[tex truckie]

hello all ,
i have a 72 super its all original.
402 cid,400 turbo,holly 650,hugger orange/white(sweet looking truck)
i get around 11.5 mpg.
i was worndering if any one might know a couple of things or have suggestions about:
1.when cruising 70 mph my tach is at 3000 rpm,of course this is not helping my mpg,is there any way to get this lowered without affecting performance greatly?
2.i am unsure of my factory gear ratios,but when people say a higher gear would help this out does that mean a lower number?
3.also i hear overdrive tranny would help,i was thinking like a 3 speed overdrive turbo 350 from a 80's model chevy,would a 350 turbo hold up behind this BB 402 or would it being to much torqe and blow it out?
all info and or suggestions are apricciated .