[shifty]

I missed the other two items.

For the shifting problem, could just be the linkage is shot or your clutch is poorly aligned. Mine did the same thing until I ended up putting a new clutch in mine out of frustration - I noticed it was out of alignment when i pulled the engine (first time ever pulling one here, and the people on this board helped me out!), and it was a little worn, so I just got a new one, aligned it proper, than reinstalled - one of the two things fixed the problem...I don't think the transmissions are synchronized in 67 and 68...could this be part of your dilemma?

For the leak, is the air on when you smell it or not? If it is, check the big bulky box on the passenger side of your firewall - this is where the blower fan is located. It's probably cracked and you have a header leaking or something. I guess it would be advisable to put a light inside your cab at night and light it up, close the hood with it in there, then get under the dash and look for light coming through. Move the light around. Try stuffing something in your rear cab corners in case they're blown out and that's where it's coming through.

EDIT: Nevermind, The light idea was already given