Growing up we had a 69 Olds delta 88 with a 455. Mileage was 16, reliably. Axle ratio was 2.56.

Here's what I see in your first post:

Gas smell when engine is running means WAY TOO RICH. Why is a fuel injected engine rich? Because of your camshaft. If the intake manifold vacuum is part of the mixture control and you are not idling with 19" of vacuum that's your #1 problem. I bet you're at 13" max.
So,
CHANGE THE CAM!
Something like 206/212degrees @50 lift, 112-114 degrees LSA. Factory EFI cams are often at 114 or 115 LSA. I'd say call Hughes for this one, most of their standard grinds are on a 112LSA. I'd say smallest Voodoo but IIRC it's on a 110LSA.

Shoot, the "small summit" cam is 204/214 @ 50 lift, 112 OR 114 LSA. It may be the budget answer, and boy do I dislike this 50-year-old Chevy lobe grind, but it'll get the job done for you. Make sure it's advanced four degrees. Many street cams are ground with the advance already in, you must check.

Now with the camshaft dilemma settled, you need to drop to the 2.76 ratio in the rear. No need to do the overdrive swap. The smaller camshaft will increase low end torque to get off the line while the 2.76s limit rpm at highway speed.

All this was assuming your EFI setup has an O2 sensor. Sounds like yours does not. Some of them didn't. If your setup does not, see if you can add one.

Ignition curve should be 38 degrees mechanical, all in by 2500-3000 rpm. Then add vacuum advance on top of that.

If your 727 does not have a part-throttle kickdown, that will help as well.

That's all for now,
R.