I swapped a 6.0L into my 86 4-door 1-ton dually...originally had a 7.4L.

I also used the 4l80E...the old trans was a SM465 4-speed.

I was really suprised...my old setup was good for about 8-11 mpg avg...towing or not.

The new 6.0 makes at least 14 to 15 mg...I'm very happy with it to say the least. (this is a big pickup!!!)