OK, seems like larry got some points there, my main question is, what do I leave stock because cost would simply be too high for the increase in mileage (no use spending a thousand bucks on cam and lifters if that saves 200 bucks gas in a lifetime) and what do I change because I will see real increase?
From what I gather from all the answers put together is:
-get a near new 350. (which I have)
-take it apart and have it blueprinted (what do you mean by that, never had a blueprint done so far?)
-put in forged pistons
-port the heads to match intake manifold or other way round
-port ex runners (headers)
-go for FI rather than carb
- what FI? Holley or Edelbrock? TB or Port?

Is this overall correct?
Why would a 305 be killed in a truck that heavy? I've seen those vans being moved with the small V6s and they run OK (but I want a V8)
In all, I'm building this engine to later save money on gas. I know many of you think I might be off my head and why don't I buy a toyota starlet if I want to save gas, but the thing is, this is what I want to do just as much as you want to get 600 HP out of an SBC. But I don't want the engine to cost all that much either.
My main problem is that almost all upgrades you can buy will get you more power, but that's not what I want, I want upgrades that will decrease consumption, if that brings more power it's OK, if not, also OK.
Have any of you tried this before? Do you happen to know other things that will decrease my consumption?
Thanks,
Mad