Be very, very careful to not let the cam slide back into the engine. It can (will) knock out a metal plug which seals off the back end of the cam galley. The plug is behind the flywheel and you'll have to pull the tranny if you knock it out. With the gear off you are in perilous territory as the gear is what rides against the thrust bearing to prevent this from happening. I've always been able to gently slide the gear onto the cam nose (clean both surfaces well first) far enough to get the nut on. This involves both lining up the gear on the cam key and the timing marks on the crank at the same time. Then just tighten the nut down. I think taking the valve cover off and removing the rocker arms makes this much, much easier as it allows the cam to rotate freely, and doesn't take long.

Don't forget the spacer between the cam and gear - without it the gear will clamp onto the thrust bearing and tear up the new gear soon after it is started.