Thanks for the Plane & Pilot specs and your calculations, abdg.
Specs for Stearmans seem to vary - depending on where you look.
This, of course, is just an academic exercise and without the proper specs for Ewald's Stearman, we can only speculate.
Despite TCT's claims that the 'Spirit of Artifice' is original, it strikes me that it is really closer to one of Ewalds's "Super Stearmans" - just minus
the upper-wing ailerons.
As to where the additional tankage might be placed, it strikes me that, in order to keep the CG within reasonable limits, the extra 35 or so gallons
would have to be contained within the inner wings - both top and bottom.
Any other ideas?