Your gears wouldn't be grinding because the actual gears never contact in the transmission - it's the gripping "dog teeth" gear that slides between the gears to connect them. But it sounds to me like:
1) Too much friction from the leak
2) Your clutch isn't fully de-clutching, thus burning out
3) Where'd you get your clutch?
P.S. If you don't have a really good understanding of clutches (although I assume you do because you had the brains to put in a new clutch), then read this article.
http://auto.howstuffworks.com/clutch4.htm
If you do know how it works - sorry, I'm not trying to insult your intelligence.
- Dzur