If it is higher MPG that you are after,
my suggestion would be:

Spend money on just the right gearing to get the throttle about 80% open at the engine rpm you are travelling at.

Spend money on aerodynamic improvements if your travel is above 40 mph.

Spend money on tires that have the lowest rolling resistance possible, with the level of safety for braking and cornering you are willing to live with.

Spend money on higher compression pistons and cylinder heads.

All of the above are likely to give
"More Bang for the Buck"
than exhaust system modifications
to improve MPG.

If you are insistent on playing with the exhaust system, then your number one priority should be:

"What is my exhaust back-pressure at different RPMS and throttle openings now?"

Buy a pressure gauge that can read 0-10 PSI and connect it to your exhaust manifold outlet with enough steel brake line to let the gas cool down and not damage the pressure gauge.

I think you are going to find that in daily driving at part-throttle the exhaust back-pressure is already low, probably less than 2 psi.

When the engine is not at
full throttle at 4000 RPM
there is just not much exhaust gas to begin with,
and reducing its pressure may improve MPG,
but the amount is tiny.

Are you willing to spend hundreds of $
to gain 0.2 MPG ?

Spend that money on tires and you might get
2 MPG.

To really do the engineering on the exhaust
you would actually need a recording oscilloscope and both a very fast acting pressure sensor,
and a camshaft position sensor,
the o'scope could read.
You would use this high $ combo to make the reflected low pressure pulses show up at your exhaust valve seat inside the right 'time window' for the rising piston to benefit from.

How to do this on the cheap with special headers?

Maybe convince a student at UNC-Charlotte
in their NASCAR engine mechanic program
to do it as his/her senior project,
with David Vizard overseeing it.
It would be a great 'demo' project to have on a Resume that would probably get that student a job with an exhaust products company.