Do the splines on the spindle match the grooves in the crank in number and width? An Octalink has 8 splines (hence the name
) and the BB-6500 I measured has splines 0.09" (2.3 mm) wide and .19" (4.8 mm) long. Check if your crank has grooves the match these dimensions.
I've never seen two different spline lengths on BB-5500 bottom brackets but I guess thay may have been made that way. The one I have in service is identical to the BB-6500 I mentioned above.