It's not connected to the idiom "to have one foot in the grave" (which means you're 30 years old or more). It's simply an expression of surprise. The basic phrase is "Well, I'll be - !" The rest is simply made up, and sounds charming and amusing.

The correct saying is "One foot in the grave". It means the person is already on the way to being dead.

I've never heard that idiom. It sounds intentionally elaborate and old fashioned, like from a western movie. If I had to guess, I'd say it's a variation on "to have one foot in the grave," which means "to be half dead," or "to be near death."