I dont have another torque convertor so the old one is going back in. While on the subject, I've read that it's very important to mark where the torque convertor sits on the shaft. Why is this critical?

I think what he meant was he marked the TC position in the bellhousing so he would know how when it was fully 'home'. (There is a gear inside the TC that engages in the gearbox) Put the TC in the box then spin it slowly while pushing it in. It will probably go back a couple of inches. Worth noting his warnings about ATF lurking in the TC and behind the filter