If your arc measurements are accurate, and I'm not questioning them, it seems to me that the inconsistent differences in arc between detents (large going to small going back to large) can only be accommodated by a shifter designed for that specific transmission, either a re-packaged OEM or an aftermarket designed to account for the strange detent spacing. As you point out, the first ten degrees of arc only needs 0.71" lateral travel, while the ten degrees at the other extreme needs 0.92". You can see that by putting orthagonal x/y lines for the two points on the arc - the higher you go on the circle the more horizontal movement is needed, which is why I suggested that the arms need to be parallel - it keeps the lateral movement and arc distance balanced between the two arms. If it were me I would be leery of ruining an expensive transmission with a re-engineered shifter, but that's just me. Hope you get it figured out.