How much power is lost through a 518

My Dart went from a 904 to a 518 (LU+OD) with no benefit in mileage. MPG even got worse for a bit.
This was with a low power 8.6:1cr 318ci and 3.23 gears in the lightweight 7-1/4" axle.
First 518 went bad, changed for A518 w/OD only.
Changed rear axle to 8-3/4" w/3.23 gears, no change.
Changed to 3.55 gears, no change.
Changed engine to 11.3:1cr 360ci, no change.

I did notice a difference in revving the engine with the smaller lockup stall convertor. The other 518 got a large late '60s stall convertor.

What DID change was the car got way more fun to drive. The added 4th, OD gear really brings an old car into the present day, by being able to drive along modernday traffic without turning a silly high rpm.
I'm currently preparing an A500 to replace the A518.