Rob,
I believe the Extremliner takes a different spline Q-C gear than a standard 10 spline. For some reason I think it is a 22 spline gear and or shaft. Out of curiosity what is the minor diameter of the shaft in the Extremeliner? I think you will be fine with that rearend. As RacerGeorge said many V8 Quickchanges rearends have gone very fast.
My Brother and I own a Modified Roadster, with a one off 1-1 QC. The QC was broken when we bought it. Seems the 10 spline Pinion shaft would occasionally sheer in two when the previous owner ran the car. The car was run with a Blown Fuel KB, which put out 3000 HP and 3000 torque. It only went to Bonneville once and with that rearend recorded the fastest 1/4 mile time ever at 296, at least that is what the previous owner told me. I have no reason to doubt him as the car still holds 3 El Mirage Records ranging from 266-280 set back in 1994. All records were set using 10 spline Q-C gears.
Tom G.