I think it need to be in neautral, all other gears would not release. Once in neautral, the side plate came out with no pressure on it.
I noted where someone had deburred/ground on some area in the interior at some point. Because the trans was UPS to me, I don't have to old oil to inspcet. However this trans looks very nice.
Lastly, if the side plate will not come out with ease, don't force it, its in some gear; you are going to start breaking something.