V8 or Slant-6?
 

When looking for a car, should I focus on getting a V8? Seems the value will be higher, yes? However, from an economic standpoint, fuel economy, one would look towards the 6?

it all depends on what you want the car for
if you are looking for a driver w/ good as mileage, go with the /6
if you can also get that with the 318
if you want more power, you go for the 340,383,400,440 etc

Slant six will get decent gas. And that is a tough engine to kill.
318 Will give you a little bit of both worlds Power and decent gas mileage.
Basically what GTX said

Plenty of high performance stuff for the six, and it would be kind of unique if done that way
http://www.slantsix.org/

X3 with what 67 GTX said.

Plus I'm not sure what you qualify as good mileage but all these cars have carbs and don't get anywhere near what a little 4 banger import gets now a days.