Jon,
A few thoughts, for what they're worth..
Two stretchers may be better for bolting on the tender superstructure? One or two bolts on each would help pull body flat to chassis.
Secondly, they've been cut now so why not use them both?
If you are concerned about weight, at least half the metal can be removed on each, I suspect, (lightening holes, reducing width or thickness between bolts etc) without affecting their structural value.
The rivets you see on photos probably relate to cross stretchers between sides, below the oval cutouts. Typically, (but not necessarily specific to the Stirling), simple thin plate stretchers with flanged ends would have sufficed. (See photo).
Assuming something similar was fitted, they would show on the model, so apart from adding rigidity would look good. Simple flat sheet stretchers with bent up ends could be soldered/ rivetted/ bolted as appropriate.