As Dave S said a 460 and good gas mileage are seldom used in the same sentence. Cubic inches take lots of air-fuel mixture to be burned to move those trash can size pistons and a 4000 pound car (yeah it's about that heavy, I had one with a 406).

I'm looking very seriously at the MSD Atomic as a retrofit for my '31 but will wait until about this time next year as there really is very little experience since MSD just started shipping units about 6 weeks ago. As far as no return system - yes, to a point, but there is a questionnaire to determine yes or no for your use They do have a new forum just for these EFI that I joined a couple days ago, so will try to keep up to date on the development and make a decision in the fall.

Another way to go is the Megasquirt EFI - and they too have forums dedicated to this system. It's really a DIY install, can be a little frustrating from what I can see (the available info is kinda scattered) but you do have a choice of throttle body or direct injection. Then there are several levels of complexity. Cost wise - probably about the cheapest if you can source and build the modules.

The other systems - Roger seems to have the good info there