t-5 help

I hate to beat a dead horse but I've searched for awhile and can't find a good answer. I have a 87 mustang and the trans is done. I have a trans out od a 02 v6 stang. I know the input is a different length. Can I use the gear that's on the old input or do I need to swap the gear. One is a 21 tooth and I think the other is a 23 tooth. Second what fluid do I put in it. I know one had dex merc. The v6 trans smells like it has gear oil in it. Any help would be great thanks.