If you want mileage, get rid of the cam and probably the stall converter
Go find yourself the modern day equivalent of the old sixpack cam
Get a vacuum advance distributor
A real world, old timey comparison:
Once upon a time, in a life a long time ago, my old '70 sixpack RR with the sixpack intake removed, and an 800 Holley DP, headers, 3.54, G60X15 rear tires, and a recurved factory dual point dist ran right at 3000 at 70 mph. That thing WITHOUT careful driving, and "hang on" A/C got just under 14 mpg back in the 70+ mph days
Later, the same car with a 71 (high compression) 340 swapped in, headers other wise stock, would get 17.5 mpg and better. That combo would run right with a stock 383 RR. In 73, I drove this combo from San Diego (Miramar) to N. Idaho to bury my little brother. Through Oregon, they had 75 mph limits then, and even at 75-80 mph, that 340 cranked out 14-15 mpg!!
I must stress AGAIN that I did not ever drive with an "egg under my foot." When I got onto the freeway, I got ON the freeway. I'm not one of those guys who gets to the end of a ramp doing 40 mph!!
This car weighed 3860 with "some gas" and a spare tire. I was in the Navy then, and typically had a LOT of weight, including two huge toolboxes in the trunk. This WAS a 4 speed rather than TF.
