If it's me, this is what I do;

* 700R4 Automatic
* GM Crate Motor - Fast burn 385 with cam kit 430 HP
* Final drive somewhere between 3.20 and 3.55

Your idea of either building your own BBC or SBC is admirable, but then then to discuss economy is a bit of an oxymoron. By suggesting economy, you're looking at the financial issues in driving it, but not in building it. A crate motor will run $4500, and more to build one, especially a stroker.

If you are really concerned about economy, I'd look at building a 3.8L supercharged engine. Easy to get 400+ HP out of using modern EFI and will get you near 20 MPG.