I have long wondered just how they are able to aqeeze so much better mileage out of engines today, when compared to engines of equal power from say early 80s. I know there are MANY factors, such as fuel injection, fast-burn heads that allow you to run higher compression, OD transmissions, etc., but I just don't see how those things add up to the huge difference. Of course, my only experience with a pre-80s car was my 69 Camaro w/ a 289 and that did get pretty good mileage. Why the large difference? When manufacturers had to detune everything to meet tougher emmissions in the early 80s, how come the mileage did not INCREASE, rather than decrease?

Wow, what a mouth full. Well the answer is at least.
Your observations are correct. If fact, the answer is what I teach for over half of the school year. So here is the short version, Very short.

Precise control. Carbs, ignition, engine machine, mfg procedures, etc. were not that tight of control, nor could they be with the technology we used to have. Now instead of the shotgun approach, we use the lazer knife approach to design and control. This extream control and precision give the advantage, and results.

That has to be one of the 'shortest' answers you have ever posted Ken...!!!

Tell me than, if i were to put a factory crate newer model motor in my 74 vette, would i get the benefits of better gas milage than i am with the original one? i know it sounds stupid, but with it being my daily driver at 85 miles a day, an extra 5 or so miles while saving the numbers matching one could be worth it.

I could have made it a bit longer Jim, but with that question I could have gone on for hours. Thought I better shorten it up a bit
As far as a crate engine getting better mileage, not really.

The specfications and build design are similar to the original engines. It would be just a bit better due to the ring designs, but very little. You may also gain a bit due to pistion design.

To get big improvements you would need to go with computer control timeing, fuel injection, and to do really well, distributorless ignition. You could do the same to your present engine.

The cam profile and compression ratio also makes a difference.