I checked my 490 Powerex 75 % mast and the IMCS is 29. The 460 Joystick is 25. Both of those masts should work well according to the Sailworks rigging guide.
My sails are 2004s and the masts are about the same age. I agree with you that the 490 mast is too stiff for the 8.0 Retro and the 460 too soft for the 6.5 Retro. However, I don't understand how come they don't work since that's what Sailworks recommends. The Joystick 460 is the ideal mast for the 6.5 Retro from their chart.