Is rear chassis rails conversion necessary?

[DRCorsa]

I have built the chassis to the book specs. I have not modified the rear rails area though. The difference to the book specs is that instead of 20mm diameter dircular tube i have uses 20x20 rectangular tubes joined with angles at the corners. Look at the attached pictures to understand.
I am a bit reluctant to cut these rails as i think that they offer some bit of extra stifness to the rear part of the chassis.
From your experience, do you think it will be possible to install the SSC rear GRP panel to my chassis? Thanks!