mid 80's 318 mpg economy numbers...

Not all 518-OD transmissions have Lockup.
My current one only has OD, so 'any' older 727-stall convertor will work.
This '60s stall convertor was the only one I had around that suited the combo.
Well, I did have a slightly smaller TCI 'Saturday Night Special' convertor. Tried it once, and the convertor-slip made the car feel like it had jelly inside the transmission.
Took it out again the very next weekend.

Mileage 'card';

DartMileage.jpg