pain in the rear!

That first shackle is connected to an extended bracket on the frame unlike the second photo that is attached to the frame only.

I like the looks of the first pic, they did a nice job fabricating that.
I think it would work in your case but the center support would have to be lower
than what is shown on the bracketed mount in the pic.

Looking at the 2nd pic, you'd have to add length (over the stock size shown) to get the support bar in and not contact the frame during suspension movement.
I'd estimate a 1/4" from the support bar from the leaf connection should be allright as a minumum.
Support bar to frame I'd give an 1 1/2" as a minimum?

As far as removal of the old one, any way you can get 2 pairs of vice grips on it and get the bolt loose then? Small 2x4 blocks come in handy on a job like that to set between the grips and body/frame.
The less welding you do around a gas tank the better trust me.

Personally I never cared much for these extended shackles, I used them in the past and didn't care for the look of them showing.
I just added air shocks a week ago, 60 lbs. pressure, got the hieght I wanted and rides smooth as a Cad. .

P.S. for member Jerry S: Here's another headline your gonna open jerry s looking for "lurid" details and again your gonna be bitterly disapointed

As a welder I can tell you anything with sparks from welding or grinding around that gas tank be carefull, and have an extinguisher handy. If you have just the smallest gas leak those sparks will find it for you.

I wish when I welded full time years ago they had that Bart Simpson shirt available that reads "I didn't do it, nobody saw me do it, you can't prove anything" (I won't go into details)

Yea, mock it up and see if you think it may work, using a stock sized shackle though I don't think you'll have the clearances your gonna need.
You could use heavy flat stock Cut a heavy gage pipe lenghtwise in half and use that Just a few suggestions.
Those original Ford shackles look pretty flimsy anyways to start with so anything new will be an improvement.