Can anybody please tell me if these 2x upright rails on the inner edges of the footwell ends were moved inwards whether it is likely to affect the torsional rigidity at all, and if so where would be best to add braces?

Your problem will be lack of foot space, provided you weld the face with some sheet steel it should not affect much.

how will you sheet the tunnel?

I would also move the tunnel pieces that join those upright rails at the top and bottom, so at least the stresses are still routed correctly (and not into the middle of a bar). That will have the positive consequence of allowing you to still line the tunnel.

But as Mark says, the current design only just leaves enough space for three pedals as it is, if you move them in too much you're going to have to get smaller feet, or maybe go for an automatic car?

Thanks for the replies.

I didn't bother to draw all of the tubes in but was thinking that with the rails inset and another vertical pair equidistant from them running rearwards I could fit a small curved panel into the corner. This would only be about 2" max so hopefully won't impact on the pedals or actually narrow the footwell.
The reason behind this is I'd like my transmission sitting further back but the bellhousing fouls these 2 uprights. With them inset the bellhousing can move further back before it contacts them.