I have a 1990 Econoline 250, it has a little leak in the valve cover gasket, it sits during the winter and the gasket contracts which makes it pretty bad the first time it starts up. After that not so bad. I know I have to get it fixed, but for the time being, is it safe to drive it with oil burning off the manifold? it's a decent amount of smoke, but not a ton.
It really depends, but most of the time you won't get enough oil off of the valve cover to start a fire. You're right, there will be a lot of smoke. Before you start it, you should try and clean off as much as possible. This will limit the amount of smoke which is produced. Also, check the oil to ensure there's enough (actually, if it's been sitting over the winter, you'll probably want to change it right off).
If it is a copious amount try to wipe off with rag (preferably when cold), otherwise will burn off pretty quick, no other issues.