I,ve just been re reading this thread & think I now understand your suggestion Baz....
Is this what you mean?........
To have a tank sat on 3mm plates spread across the gap between RS14 & The GRP1 tube?
To remove the two D13, diagonal tubes? & extend the tank along the length of the GRP1 tube? ( width of the car )
To attach the GRP tub to the diagonal tubes ( I presume you mean D11,s) behind the suspension boxes?
To join the back brace bars of the roll bar to the top of the GRP tub support rails?
If this is your plan........By removing the 2 D13 diagonals then the only thing supporting the tank would be the 3mm steel plates (CP8,s) & the 2 spars GRP2,s & the long spar GP1, this wouldn,t in my opinion give enough vertical support to the tank.
With regard the back braces from the roll bar to the upper tub support rails......when using a GRP rear tub there is no upper support frame for the tub so the back braces from the roll bar need to go from the roll bar all the way down to the chassis base rails ( in my car a steel plate welded in between RS14 & GRP1), tho this would give the tank position in your suggestion alot more support.
If you intend to use the "Book Spec" tank dimensions, just be aware that it won,t fit when using a GRP rear tub.
Sorry for the waffle/ramble, just trying to understand your suggestion, it would work if you took the roll bar back braces all the way down to the chassis rails to a steel plate between RS14 & GRP1.
Hope I make sense?
cheers
andy