OK, how hard is this? My truck leaks oil and it looks like it is coming from the front seal. I know the pulley is pressed on and I need a tool to pull it, but what is the best way to put it back on??? Is their a woodruff key on the shaft?

Yes the crankshaft damper has a key and key way on the end of crank.
Run a search on Kent Moore tools. The people that make GM special tools.
They make a crankshaft damper remover and installer tool.
J23523-F.
Also a seal puller and installer helps.
Any good 3 prong bar type puller will pull it.
The hard part is getting the pulley back on it is a press type fit.
In the good old days I have used a big hammer But A time or two I have saw a big hammer snap off the end of a cast iron crankshaft.
Another note is if the old seal has put a bad grove in the pulley sleeve they make a drive on ready sleeve repair kit for it.
Ask your parts place about it.
Good luck MT
And remember Proper tools make the job a lot more easy and better.

Autozone has a free loaner program for the special tools needed.