I will try to check my pictures of my one over the weekend but the hub carrier looks like a Cosworth one to me. If that is the case, then a 273mm rear disc is what you need. If you can measure the gap in the caliper carrier (where the disc sits inside and the pads sit on) to see if it is around 13mm or 24mm that would help as that would determine if solid or vented discs.

the one real differences are the hub carriers and the brake caliper carriers between models and solid or vented calipers