comparing my old 904 to the A500 in my 5th ave, dry w/o torque converter or fluid, I'd guestimate the weight difference at about 70 lbs. weight differential between a 727 and 518 should be about the same. internals aren't drastically different in the front half, and the OD planetary is just another 727 gear cluster. if they were built equally (same converter, both rollerized or non rollerized, both steel or aluminum drum, etc), I'd wager you won't see a significant difference in E/T...MAYBE 1 or 2 tenths. I could see maybe another .1-.5 more maybe going from a 518/727 to a 904, because they are much smaller internally and take about 1/3 the power to spin.