charlie is spot on with his advice. The plates can be "stuck" pretty good after years of old oil, paraffin wax, and you name it. Gunk likes to build up behind the plate... always best to remove those things when you are doing this kind of work.
A little tip: take a blunt chisel tip and place it on the edge of the oil slot (this should be clocked at about 12 on your plate bolted to your block). tap a little sideways, each way, and the plate should come loose. You could also try a pry bar to lever it out too, but if you are going to reuse your old one (sounds like you might need to replace it) don't warp it.
I "think" the hold down nut size is 1 and 1/16"...
Daniel JessupLancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com