Whoa, Danny! ....
This sprung WLA solo seat idea quickly expanded to a LOT more work than you first thought. Before you contemplate such drastic measures as frame lengthening/butchery, think more along the lines of Keeping It Simple.
Let's back up a little. First up, the WLA-inspired seat look you're after should be fairly simple to achieve. You'll need one of these hinged solo seat front mounts (hundreds of these on eBay). The WLA pan should already have bolt locations for one of these.
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Then you could adapt the underside of that WLA steel seat pan to accommodate a pair of steel rails to fit the kind of mounts normally found on modern sprung seat posts. Shown in this pic. 'Cept maybe snug the rails closer up to the pan so they aren't so visible.
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Easiest way to do this is to get your hands on an old, cheap saddle and cannibalize it for the rails. These are commonplace. Weld the rails onto the underside of the steel pan - and you might just want to start with a bare metal WLA pan rather than rip the leather off an already covered pan. I've seen bare steel WLA pans on eBay at cheap prices..
My only concern about your WLA seat plan would be that its a motorcycle saddle and its size might overwhelm a bicycle. Not saying it will. Just something to be aware of.