Found four 3/8" UNF bolts and got the tank fixed down on the new brackets, the rear one still needs that little bit of alteration to allow the option of sliding the tank back by 1/2". Although it had to happen, because both brackets are equal, really pleased that the bottom of the tank faithfully tracks the lower frame rail. An unexpected bonus is the similarity of the curve of the tank front to the web of the frame casting, like to say I thought of it, but it ain't the case.
This is the start of the seat pivot, all done with smoke, mirrors and ty-wraps at the moment, but the foundations are laid. Using the original pivot boss was a no brainer, no way in the world was that gonna get cut off to be replaced by something else doing the same job, a little wider than I'd like but it's workable. What was definitely not going to happen was adopting the original Meriden idea of the seat nose going above the bracket with vertical pivot links facing downwards from the seat frame.
Thinking of all kinds of stuff for the pivoting mount, 1/8" flat, round bar, whatever it had to be set in to come inside the nose of the seat. Then the idea of a coupla ring spanners, pre-set and forged when made, they've got a built in factory look already. They're a pair of 1/2" AF spanners with hexagon sleeved huts brazed in and tapped 5/16 cycle.
Weld a bridge across the seat end and it should be good to go.
Another look at the top pic shows that the springs are gonna take some thinking about, longer than you'd like, to say the least.