You might want to check your drill press to see if it uses a taper fit chuck.If so then there is a good chance you will drop the chuck on the work piece during the process. Taper fit chucks are designed for vertical downward force. Not lateral. If you are going to go for it anyway then for sure use a four flute end mill. If the work piece is a 16th" thick or greater then make a couple of passes rather than trying to hog it all out in one pass. Don't ask me how I know this. :rolleyes:
Some good advice came in while I was typing. Yes! the compound vices work well for this. My first "mill" was a floor standing drill press with a compound vice bolted to it. And yes, it had a taper fit chuck. It became a bigger pita than it was worth. I ended up buying one of the mill/drills from Harbor Freight with about 20" of cross travel. I have cut slots as small as 1/16th" with it in steel.