Sherri,
Model 66s do not drop the feed dogs, so you'll have to find either a cover plate, old OEM or new style or try it with the stitch length set at 0. I think they call them darning plates. They look like this:
[ATTACH=CONFIG]486470[/ATTACH]
What SteveH said about converting them. I have unconverted one Singer 66 treadle and would do it to any of them.
There are MANY electric Singer 66s in cabinets to use without (in my opinion) ruining a treadle machine.
I can treadle and up until 2 years ago I'd never sat in front of a treadle machine before. If klutzy old J Miller can run a treadle machine, you can.
Joe