I'm having the same kind of problem. I bought a new aftermarket shift handle and it seems to not fit where the old one goes in. But after inspecting it, I notice that the roundness at the end of the newer handle is a little wider than that of the older handle. And the slot that it fits into is round also, but I guess the new shifter is a little too wide. So the best thing to do is probably grind off a little at the end. Haven't gotten to it yet, but seems to be the problem...
JAA