I vote gears and the 10:1 - 327.
My '64 with a 327, th400 and 3.73 rear was a blast to cruise around. No where near 10:1 but it had headers, intake , full msd hei, dual 3" exhaust and a shift kit. Rpms depend on cam and heads, who knows what your 327/350 is capable of. My 65 gets 22mpg with 700+hp. It's at a neck breaking 55mph but it's all on the heads and cam and how much vacuum your pulling. 3000 rpm and about 55-60 mph I have 22 on the vacuum gauge. The truck is shaped like a barn door.
Put the motor in and have fun

I can explain it to you, I can't understand it for you.