I did the 904 -> 518 swap.
Although I wished I had used a 500, they are hard to find here.
The 518 is a heavier transmission (internals) and that has an effect on mileage.
LockUp or Non-lockup is only a convertor issue.
I have used both types in my Dart without issues.
The Lockup-version comes closest to a 5-speed transmission.
I'm running a non-LU currently with 3.55 gears in the rearaxle.