Then there’s “Well suck me dry and call me Dusty!”
Not terribly clear about that one either…
I’ve always said, “Well smack my ass and call me Judy!”… but I have no more of a clue how that saying came about than anyone else here does. I do love the responses I get when I say it, though. 
Just a little peril…
Here’s mine:
Well Crap in a bucket and call it Gravy!
Not quite the same, but my favorite is “Well, I’ll be dipped in bunny shit!”
I’ve always heard it as “Well slap me silly and call me Sally.” No idea where it originated from. I’m gonna start using the “butter my buns and call me a biscuit.” 
Well, dip me in honey and throw me to the lesbians!
(or my favourite:)
Well, slap my face and call me Zsa Zsa!
Question marks and exclamation points may go inside or outside of quotation marks as needed. Have fun.
Really? Well dip me in shit and call me Hershey!
My granddad was a cattle farmer in Oklahoma and he used to say “well, slap me silly and shave me naked!”. Always made my chuckle.
Not just the South. “I’ve got an old mule, her name is Sal / Fifteen miles on the Erie Canal”
My Daddy always said 'Kiss my ass, and call me stupid"
my goto phrase is “Well, fuck . me . sideways …” (in a somewhat weird, drawn out southener’s accent)
I like its compactness, no calling needed
… especially when resurrecting a zombie
It’s a known formulation on this side of the Atlantic too. My mother would sometimes express surprise with “Well, bugger me gently!”, and in a completely different context I’ve heard, as a sarcastic remark on someone overfond of leather-and-chains fetishwear, “Well, whip me, beat me, call me Betty!”
In my part of the UK you used to hear “Don’t piss on my shoes and tell me it’s raining”
Other urine related expressions being
“I wouldn’t piss on you (even) if you were on fire”
“I wouldn’t give you the steam off my piss”
TCMF-2L
Courtesy of Unca Cecil:
I wouldn’t piss on your ass if your piles were on fire!
I still remember every word of that song from elementary school.
Didn’t Bob Newhart say the slap-Sally bit in his standup routine?
[Of a particularly ugly face] “Like a bulldog licking piss off a nettle”
Blockquote
That’s a familiar one now you’ve reminded me.
Other ‘face’ ones include “Face like a slapped / spanked arse” and “Face like a welder’s bench.”
TCMF-2L
‘Well, stap my vitals!’ Is an exclamation of surprise from around 1700. I wonder how far back this form of speech can be traced. Is ‘bless my soul’ older?