I'm attaching a photo of the connections on an old range with a 3 conductor plug that I want to convert to a 4 conductor plug. What looks like a strap under the center of the connection block is actually not -it doesn't hook to anything nor is there a screw hole for it to attach to.
http://www.friendlyspider.com/stuff/range.jpg

The equipment ground jumper is the one from the center terminal on that white block to the right, from center terminal to the frame of the appliance.

Remove that jumper, and connect the green equipment ground of the new cord to the frame.

By the way, thanks for the photo, photos help greatly.

That's what I was thinking. The white block connects to the oven element and the center green jumper from it is the equipment ground. So I will disconnect it and hook the green wire from the new 4 conductor cord to that screw on the oven housing. Thanks a bunch.

Got one more question. I opened up the back for cleaning out some cobwebs and I found 3 more green wires attaching to the metal housing. Looks like they are from the burners and the upper oven element. I am guessing I need to disconnect these as well? I'm starting to think that maybe I shouldn't trust the old wiring in this oven -it looks intact but it's that old asbestos or cloth insulation and there's a bit of rust on some of the contacts.

Any green wires connected to the frame of the unit can remain, as they will get grounded once a new circuit with ground is installed.

As far as the age and condition of the unit, I have to leave that to your judgement. I can tell it is very old or corroded from the photo.