I put the 2.47 in my 1981SD and have never looked back. Yes, for the first 10 feet it is slower off the line, but it makes all the gears taller so it stays in them longer.. I think overall it's just as good 0-60. the 617 makes the most torque at 2450 rpm, and that's where it sits most of the time with the 2.47
Read this thread.. http://www.peachparts.com/shopforum/showthread.php?t=115918

Good luck..