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The 518 can in no way rob more HP than a GV. The 518 only adds roteing weight in 1st 2nd and 3rd, no other "drag" is added, the pump is not doing any work holding clutches or bands or line preasure or any thing. The GV adds a similar amount of rotateing weight in all gears combined with an almost useless extra gear. The OD section can easily have clutches added, if you use the 48RE OD section it can handle 800 lbs tq easily!

As for the 1 second difference in ET from a 727 to a 518 I say BULL CRAP! Like I mentioned earlier, there is a few pounds added to the rotateing weight, thats IT! In first second and third there is some extra weight and in no way can those few extra pounds knock off 1 second in ET. If the trannys get equal prep it can't be more than a couple tenths. If 5-10 pounds in rotateing weight could really knock off 1 second then every car could switch from a steel drive shaft to carbon fibre and gain a second

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Show us the math on the "worthless extra gear". I'll even give you the parameters. 3.73 rear gear, 28" tall rear tire, 4000lb car.

Also has anyone factored the cost of modifying the trans tunnel or the loss of perceived value with a hacked up cross member?

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the worthless extra gear comment meaning this:

3rd (direct) at 75mph with 28" tires and 3.73 assuming no converter slip: 3350 RPM, mist likely 3500 w/converter slip

gear vendors OD drops it to 2650 RPM no converter slip, most likely 2800-2850 with slip

518 OD drops it to ~2320 RPM, with no converter slip. you won't have converter slip if using a lockup converter...

IMHO a .78 OD is not enough OD to warrant the cost... [/quote}

Cost aside - 700 rpm drop is not worthless. The 518 is better from that standpoint, but what option do you have if you don't want to hack up your car? Keisler GM adaptation? Better first ratio and as good or better O/D ratio.