How much power is lost through a 518

i have a friend who installed a 518 od in his 3.91 geared 318 dodge 4 x 4 truck...mileage did not improve.
as for power loss to 518 from a 727 i don't know but 904 to 727 i have read may be 20 hp or more and 35 lbs heavier.Lots of guys going to 904's.