Tom, I don't think the wording has to be anything other than your wishes. Just be specific, very specific, about your wishes. Do you want your brain to be donated while your still artificially alive, meaning if you are on life support? Or do you specifically want it done post mortem? Just be specific and it should be fine. A medical dictionary would probably come in handy for you to be even more specific.

When Jim and I had our living wills done we had very specific instructions. We hired a lawyer to write ours up. We told her our wishes, she wrote it up. We had our living will, power of attorney and final will done for $250 total.