hi.... i did a similar fix a few weeks ago, following a post i found here i did it slightly different
the splines actually go all the way through the inside of the aluminium tube, only the ends wear..... so, i cut the aluminium rod in half (with a hacksaw) cut the worn ends off, re-yatched the 2 bits of aluminium rod and joined them with a section of 15mm copper pipe.... which i drilled and bolted through both sections of aluminium rod. works great.... and you dont need to remove the seat from the car