How Much Horsepower Required to Cruise Comfortably at 70 MPH?

Volumetric efficiency is a measure of the volume of air the engine is ingesting compared to the volume of it's cylinders. If you measure 750cc of air going into a 750 cc cylinder, that would be 100%. But if you measure only 600cc that would be 600/750=80%.
The VE changes constantly with throttle opening and rpm and load.
It is usually measured at WOT, to help indicate what the engine is capable of.
It is possible to manipulate VE to the better in one part of the power curve, at the expense of another part of the curve..
On a given engine, the things that affect the VE to the greatest degree are the head-flow, and the cam. This is why it is often said that the power is in the heads.
Once the engine is assembled, it pretty much is what it is....... unless you choked the carb selection or restricted the exhaust.
VE is usually greatest at or very near to the torque peak. If you move the torque peak (with camming) you may move the rpm of best VE, and you may change the numbers.
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Fresh air into the carb as opposed to feeding it 400* underhood air, will change the density of the air, as will changing altitudes. These will affect the power, by varying the number of oxygen molecules being ingested,and the Effective cylinder pressure, but probably not much the VE, because a quart of air is a quart of air, regardless of it's density.
The hardest job an engine has is finding oxygen, so anything and everything you can do to make the job easier is gonna affect the power it makes.