I have a 518 in my 92 D150 however its behind a relatively stock 5.2 ( 318) that runs low 15s, Last year I pulled the trans and had a M/P shift kit installed as well as having the stock lockup converter redone by Lenny at Ultimate the results are a tenth reduction in 60 ft times and a nice hard first to second shift as well as crisp shifts 2nd to 3rd. A number of years ago I changed the rear from 3.55s to 4.10s with the overdrive it still cruises at low rpms. When I`m racing it , I take it out of overdrive.