I picked it up at a yard sale and it rather good sized, and I'd like to use it as a planter. My husband says it is impossible to make drain holes in it

I would think using a punch to first dimple where you want the holes then a good drill bit to drill out the holes. While you're drilling add some 3-N-1 oil (do they still make that) down the shaft of the bit which will make cutting the holes easier. Also, drilling at a slower speed will help.

Or a work-around could be to put your plant(s) in a shorter plastic pot and set that on rocks or bricks to allow extra water to sit in the bottom of the pot rather than the bottom of your soil. If it rains real heavily, you could lift the plastic pot out to empty the surplus. Wrap the plastic pot with spagnum moss so the edge doesn't show.

I bought my husband a hammer drill for father's day last year for just such tasks